"Two" brought to mind the Dewey vs Library of Congress call number systems. I don't know why; I know there are other systems. In fact on my campus alone, although we use LC primarily, we also use Dewey (for a lot of the education materials), Moys (for law), first-three-letters-of-author (for special fiction collections), super-local accession numbers (for a lot of AV material though I think these at least are to be reclassified at some point) and so on and so forth (standards; product catalogues; archives). And I've never understood the "ringed" classifications at all. All of which is surely enough to make anyone emit a hearty existentially anguished "Why?"
But it's the LC vs Dewey that I mostly have to explain to new students familiar with school and public libraries. And it's not that I think that one or the other or both should change, because I understand their respective niches. But those poor students. Really it's no wonder they ask for books by bib number, or ISBN, or author, or colour.