Can there be a difference between learned and learnt?

I notice that in your example, 'learned' is followed by a consonant and 'learnt' is followed by a vowel. It is a very small sample but, examining my own speech patterns, that reflects the way I speak.

I draw no deep conclusion from this because research would be needed to see if this is a real phenomenon.

Maybe there was in the Distant Past...-

There may have been a difference a long time ago. My cursory research* for this answer made note that this is a particular category of irregular verb, which according to Oxford Dictionaries is any verb that does not follow the usual -s (third person, -ed (past tense), -ed (past participle) and -ing (present participle) tense structure.

Englishpage.com calls this particular variant of irregular verb T-Forms, and notes that the ones without a vowel shift like 'learnt' are slowly being dropped from the language altogether, which to me that suggests it is an etymological vestige. I never really seriously studied etymology, so I am not certain exactly what that vestige might be but The Wikipedia's page on irregular verbs, makes an interesting note:

The past tense and past participle are the forms that are normally made in irregular fashion. About 200 verbs in normal use have irregularities in one or other (or usually both) of these forms. They may derive from Germanic strong verbs, as with sing–sang–sung or rise–rose–risen, or from weak verbs which have come to deviate from the standard pattern in some way (teach–taught–taught, keep–kept–kept, build–built–built, etc.).

It seems like T-forms always originate from Germanic weak form verbs.

Noting that T-forms always seem to have two past tense variants, I was curious if there could be any special, yet lost signification. Looking at Wikitionary's etymologies, if I had to guess, I would suppose that the distinction between -ed and -t:

-ed Etymology 1

From Old English -ode, -odon (“class 2 weak past”), from Proto-Germanic *-ōd-, *-ōdēd-.

Etymology 2

From Old English -od (“class 2 weak past participle”), from Proto-Germanic *-ōdaz.

Etymology 3

From Old English -od (“adjective suffix”), from Proto-Germanic *-ōdaz. While identical in appearance to the past participle of class 2 weak verbs, this suffix was attached directly to nouns without any intervening verb. Compare also Latin -ātus.

{Take these with a grain of salt. The 2010 edition of Random House Kernerman Webster's College Dictionary lists similar etymologies but notes they are disputed.)

-t Etymology 1

From Middle English -te (“preterite ending”), -t (“past participle ending”), from Old English -de (“first and third person preterite ending”), -d (“past participle ending”), from Proto-Germanic *-id- (“preterite stem ending of class 1 weak verbs”) and Proto-Germanic *-idaz (“past participle ending of class 1 weak verbs”).

I suppose that based upon these etymologies that these words could have some special signification in a the more complex verb tense structure of Middle English. However, I lack the prerequisite knowledge to parse the guides I have seen and I doubt it due to a little further research into middle english below:

... but there is Not One Now.

Trying to research Middle English, I came across this article Grammar in Early Modern English by Edmund Weiner, deputy chief editor, of the Oxford English Dictionary states that all weak verbs using the -ed suffix words used to be pronounced as a separate syllable, like beloved but that the practice was mostly discontinued during the 16th century. The few exceptions that remain usually have a t or a d as the last consonent or are devoiced into t, such as locked or nicked Contemporary English only has a few vestiages of the practice leftover from phoneticized spellings of the word. Some words do seem quite difficult to pronounce without devoicement. Learned is not amongst these words but it seems revealing to me to know that the more popular spelling has always been learned, especially during the questioned period. Given that this doesn't have a vowel shift either, I believe that learnt is just a misapplied orthrographic convention that gained some limited popularity.

Progressing further down the timeline is The American Dictionary of the English Language, by Noah Webster, the edition of which I'll be using is the 1844 edition, hosted on the Emily Dickenson Lexicon website. It has both past participle forms of the words as the same entry, with no special usage footnotes.

Noah Webster often included his opinions on how words should be properly used, like how "that" is often considered a conjunction but is simply what he calls a substitute for a sentence; how a clause or how the word huge should not be applied to space or distance; how "cannot" is "usually united" but "without good reason" and even when the "adjective" "An" should be used over "A", which is more noteworthy because he notes pronunciation relations between words.

The point I am trying to make here is that I believe he would have noted a difference if he perceived one and he would have been the person most likely to perceive a difference during his lifetime. However, as I already said, he did not, so I deign it unlikely that anybody did. The shared entry is as simple as can be and the happens to be one of the shortest and dullest entries in the book:

"Obtained as knowledge or information."

Learned can be affixed into learnedly but nothing else is noted about the unaffixed form. It's interesting that his pronunciation guide notes no difference for the adjective form of the word but in being a vernacular speaker of the language, I have no idea if that means he always recommends two syllables, one syllable or if that can be read liberally. Perhaps those of you wittier and more interested in the matter than I am can make sense of his pronunciation guide, which I gleaned from a scan of his dictionary hosted on the "Internet Archive" website. However, I believe this is besides the point since it is the same sense of the word that is being examined.

For a more truly contemporary reference, Oxford Dictionaries has a comparison page called *'learned' or 'learnt' also notes no particular differences between the particular variants of the word you have asked about. I am fairly certain if there were any at all now, they would make mention of it too. When they make the comparison between indefinite articles in 'A historic event' or 'An historic event' they do make note of how the pronunciation of the following word comes into effect and in their 'Cannot' or 'can not' article they do note that it is more usual to use 'can not', for instances where the word 'not' is part of a separate construction. I should also note that the 'learned' or 'learnt' article is what highlighted irregular verbs for my earlier musing.

The 2010 edition of Random House Kernerman Webster's College Dictionary simply lists the -t suffix as a variant of -ed with some T-form examples. I do not know if they would have listed any different conventions. Also, most modern dictionaries seem to reserve the separate two syllable entry learned for the erudite's adjective and simply list learnt as an alternative variant of the past tense/participle of the word learn. The dictionary I will use to demonstrate this is the fifth edition of the American Heritage Dictionary, because it has all three and it is the first listed result in the free dictionary by Farlex. See learn, learned and learnt. No signification is given.

As for why the subtitles are inconsistent, it might be an error of convenience, since there seems to be no appreciable difference to people reading the subtitles or they might be utilizing some sort of automated subtitling system, like what Youtube uses. If they are using an automated system, those can make incredible errors but you don't have to take my word for it: Do read Harvard and M.I.T. Are Sued Over Lack of Closed Captions, written by Tamar Lewinfeb and published Feburary 12th, 2015 for the New York Times' website.

I think the absence of clear evidence serves as strong evidence of absence in this case: There is no present difference, except in the pronunciation and commodity of the words.

I believe those spellings have to do with the way the human mouth is structured. After all, writing reflects speech. Not the other way around. First was speech, then was writing.

The stop consonants /d/, /t/, and /n/ (EDIT: sorry, /n/ is a nasal sound) are articulated in the same place in the mouth, a place called the alveolar ridge. All three (EDIT: /d/ and /n/ are voiced, /t/ is unvoiced) consonants are voiced consonants, which means that your throat is going to vibrate when you start producing a voiced sound. When the tongue touches the alveolar ridge to produce those aforementioned sounds, and depending what sound follows those sounds, some sounds can get assimilated, omitted, or even glottalized. Those processes are found in any language, not just in English. Words are to be pronounced in sentences after all, not in isolation.

You learned their craft...

Since the sounds /n/ and /d/ stand near each other in the word "learned", and /n/ and /d/ have the same place of articulation, a choice has to be made: In order to make a smooth transition from the word "learned" to the word "their," one of the sounds (/n/, /d/) has to be dropped. This sound will be /d/. Why? As you may have noticed, /n/ isn't a stop consonant unlike /d/. When you produce the /n/ sound, you can keep on producing this sound utill your tongue is tired. This makes the transition between the words smooth and easy for the tongue. If you want to produce the /d/ sound on the other hand, you HAVE to release the tip of the tongue from the alveolar ridge. If you don't pronounce the /d/ sound, you can jump right to the /th/ sound without any delay. Your tongue doesn't have to flap to release the /d/ sound. Thus, the speech flows smoothly.

The reason for the spelling? I think it's written "learned" because the next word starts with a voiced consonant and you have a tripple cluster with voiced consonants, namely "n(d)th"

Now for the next clause:

... and you learnt it well.

The reason for this spelling? The British don't have the flapped T sound. They don't flap their Ts. You can read about the flapped /t/ sound on Wikipedia. Since "learnt" is a British spelling and a British pronunciation, the full /t/ sound will be used, as the sound after the /t/ sound is a vowel in the word "it". You have the "nti" cluster, which is easy to pronounce.

I hope this helped.