It doesn't actually take anything away... it is just a quirk of the interface. It refreshes when you upvote, and between the time that the post was loaded and the upvote was clicked there may have been changes in the post's value. Usually that's a result of votes coming in on other posts (reduces the % share that the given post receives), or the price of steem dropping (reduces the rewards of all posts)... sometimes it has to do with downvotes coming in during that time, but that's more rare.