Well, one person who votes for everything only contributes one vote. Yes, one hated vote. Noone cares about that person, really, but when you have a really good upload, they also vote. And noone gives them any credit, as they don't really need any. If we think of them as a whole, however... That can make a big difference. Luckily, there are more sensible people out in this world than those vote spammers.