Problem: many very popular posts in terms of # of Favorites receive a small fraction of that same number in Upvotes, relegating them to not display on the Popular page. This presents a distorted view of what's popular on e621 to the userbase.
Solution: take the approach nearly every other social media website uses, by providing just one "upvote/favorite/heart/star" functionality. Why have separate functions? It's clearly not being used effectively.
Updated by bitWolfy