I'm a native English speaker with a degree in linguistics, but I didn't appreciate how hard "some/any" are for native speakers until a Polish colleague asked me to explain it, and I pretty much failed. It's something you learn how to do intuitively, but when you try to formulate a consistent set of rules explaining why you choose one over the other, it turns out to be pretty difficult.

A decent amount of English is full of inconsistencies like this. Every language has its warts, of course, but like you I never really realized how difficult English is as a language until asked to explain certain behaviors to non-native speakers.