It is not because of your vote that his winnings came down, it is simply because the value of steem, which is what you are awarding this content by giving it an upvote, is constantly fluctuating. Usually only by a little but it could be a lot. Assuming you are new to this, your vote should currently be worth less that 1c. That value will increase the more you vote on content that becomes very popular.
Also, you will notice if you get a payout that not all of your money is spendable. 25% of the money goes to the curators (commentators and voters) and of the remaining 75%, 50% becomes a stock investment or steem power. This steem power makes your votes more valuable.
Here's my article on where the money comes from.
https://steemit.com/steem/@beanz/where-does-the-money-come-from