It’s actually because “per” is used to denote the maximum “proportion” of an element. Compare “oxide”, which is just one oxygen. “per-” is also used in chemistry in the sense of the Latin prefix “per-”, which attaches to adjectives and verbs and such to convey the sense of “very” or “all the way”. For example, sometimes we refer to molecules as being “perdeuterated” when all hydrogen atoms have been replaced with the heavy isotope deuterium.