Happy New Year!
My gist. Linguistic variants (no matter how deviant they may appear to be)
are still rule-governed. We might not be able to understand the rule or rules right now; nevertheless, there are rules. To adopt your example,
"between you and I" expresses, for some speakers, e.g. a sense of politeness that could not be conveyed by "between you and me",
And, yes, 'it doesn't matter that not
all speakers extract that meaning'. Either the speaker knows (ahem, via innate knowledge) that there's a nuance in meaning and therefore uses both (regardless of whether 'to some speakers, it's irritating') or the speaker doesn't know there's a nuance and uses one or the other, never both.
The fact that a speaker uses both tells us there's a rule operating there somewhere. The fact that some speakers don't use both tells us they haven't learned the rule yet.
They're... behind the times, sort to speak.