I thought it'd be the opposite way around. Faves are public, upvotes are private. Faves have a max cap, upvotes don't (though granted it's a super high cap).
Especially on posts with, let's call them "niche fetishes" that will attract more downvotes, people who fave don't always give an upvote to counteract the downvotes and will let a highly faved post fall into a negative score.
I always apply both and used derpibooru before coming here which automatically upvotes a post when you fave it, so just wondering what's stopping people from upvoting posts they fave.
Updated