I'm guessing, but I think they favor the people with paid accounts.
Also, I often see that they leave a mediocre meme on the first 2 pages for DAYS. Obviously, it will accumulate votes that way.
Also, some people, if someone upvotes their meme they will go to that person's page and upvote and comment on EVERY meme. Some of the memers with the top numbers will upvote and leave a comment on EVERY meme on the first few pages of Hot. So if some of the people THEY upvote turn around and upvote+comment on every one of THEIR memes, it boost their numbers even more. (I.e., Smith, Jones, Black, and White each have a front-page meme. Johnson upvotes and comments on EVERY meme of each of them. Jones and White reciprocate by commenting and upvoting EVERY meme of Johnson.)