This is because of two things;

1. Steem value fluctuates up and down.
2. The upvote is not a "fixed amount" until voting ends on the 7th day and it pays out. Instead it is a share of the total upvotes on Steem during that period. This is based on some complicated math that I can't really explain (if you're good at programming it's in the source code https://github.com/steemit/steem)