Divergence in meaning of "just about" between UK and North American English

I am Scottish and we always use "just about" to mean "nearly, but not quite". E.g. "I just about passed my exam" means "I got close to the pass mark, but didn't quite make it."

A straw poll of some of my friends indicates that Australians, New Zealanders, the Irish (both Northern Ireland and the Republic of Ireland) and those from North America (Canada and that USA) also share the same meaning as we Scots. Amongst my English and Welsh friends, I have found that only those from the north of England (Northumberland and Cumbria) share our meaning.

An interesting fact I've noticed that is that everyone says "just about to", to mean "nearly, but not quite", e.g. "I was just about to leave when the telephone rang", meaning "I had not left, but was about to, when the telephone rang".

On separatedbyacommonlanguage.blogspot.it¹ (Observations on British and American English by an American linguist in the UK) we can read the following explanation:

The translation problem in just about isn't just about just. Let's think about about. The (UK) Collins English Dictionary gives us this sense-definition, which is not to be found in the American Heritage Dictionary or Merriam-Webster:

about
13. used in informal phrases to indicate understatement : I've had just about enough of your insults it's about time you stopped

Aha, the famous British understatement. Rather than saying I've had enough, you put an about in to soften the blow. And then a just to soften it more.

For more details, see the site I mentioned, which seems quite well done.

¹ _{Probably, from other countries, you have to change 'it'.}