The reason is that although an upvote may look like an upvote it is actually based on the amount of steem power that person who upvoted you has, as such if a bunch of people with very little steem power upvote that might be one cent. Where one person could come along with a lot of steem power and upvote and now all of a sudden there is 10cents on the post. That is why :) I hope this explanation helped.