Basically, not everyone's vote counts the same. If you are voted on by someone with more Steem Power, their upvote increases the reward you get by more than someone with less Steem Power. There are other factors too. Every time someone votes, their voting power goes down. It slowly goes back up again. This is to discourage someone from just voting on everything to try and get a reward for themselves for voting. Also, if someone upvotes you in the 1st 30 minutes, you get a little more reward and they get a little less reward for voting for you. There are many other factors too that effect how much someone's upvote effects how much your reward is.