Why is "second to last" not documented with the meaning of "third to last"?
There are two competing logics at play, rank and counting from the end. Those logics, combined with the specific usage next to last, result in applying second to last to mean antepenultimate.
Logic 1: Rank
Conventionally, second to last would count down from last:
- Last
- Second to last
- Third to last
A second to last option is literally second only to the last position. That is the most common understanding of the phrase. We don't really say first to last, perhaps because it imposes a logical absurdity (if one were more last than last, one would be last).
Logic 2: Counting from the End
In many of the examples you show, they count backward from the end and treat the end as a standalone value. For example, the terms ultimate, penultimate, and antepenultimate form this kind of scale:
- Ultimate (the last)
- Penultimate (nearly the last)
- Antepenultimate (before nearly the last)
These words aren't strictly counting, but they illustrate how one could count using ordinals:
- The last
- First from last
- Second from last
I use from here to indicate that the counting has shifted. This is no longer A to B (what A is to B - second to B, third to B, etc.) but A from B (how far A is from B). The use of second here remains valid, but the method of counting has shifted from a measure of rank to a measure of positional distance.
Why To?
That still leaves the question of why to may be used instead of from in your examples. Notice the use of next to last. Next to is a very common collocation, and next to last is a common way to count from the end (Logic 2), even if it uses to (Logic 1). If someone has started counting from the end this way, and then they have to say what comes after next to last, then it may sound better to writers or editors to stay with the to and continue the count than to switch to second from last or abandon the ordinal entirely:
- Last
- Next to last
- *Second to last
Consistency with the preposition is likely why the usages you present occur with next to last and why switching to second from last after next to last is much rarer.
Why Don't Dictionaries Document This?
What I've just elaborated is focused not on a single word but on differences in usage and semantics centered around one specific use case (next to last). Many dictionaries (including the Oxford English Dictionary) don't list second to last, second from last, or even second last. To the hard-nosed dictionaries, such phrases are beneath the notice of a publication focused primarily on words. So higher order notes on phrasal usage are also likely to be absent.