@kfogel Interesting observation, although I think it really can't be separated from the shift in meaning of "a couple" from "a set of two" (synonym: a pair) to "a small number of" (synonym: a few).
There is nothing grammatically uncomfortable about the contrast of singular article & plural count in "I'll make a few changes in a few hours."
But, 🤔 "few" is an odd one out, maybe: originally a pure adjective ("it takes few hours to make few changes"); the "a" later added to match "a lot" etc.?