Early upvotes count way more than later ones. They have more power to move the post up or down. Secondly, a post will need several votes to get into the "rising" or "top/this hour" queue where the post will be visible to other users who will then upvote the post.
Remember Unidan? He only used 5 alt accounts to give himself an edge.
Turns out Reddit's favorite scientist wasn't quite as popular as everyone thought he was. Ben Eisenkop, better known as Unidan, was banned by the site last night after the administrators there discovered he was faking votes to make sure his posts got more attention.
Unidan became known as the "excited biologist" for regularly explaining natural phenomena, for being hugely active in the /r/AskScience subreddit, and for generally being a likable, knowledgeable guy. He was also five other guys, apparently—turns out he was using at least five accounts to help destroy his enemies and to artificially pad his own comments and posts.