Steem value at the exchanges fluctuates and was down last couple of days and going up last 12hrs.
Also, reward pool distribution may influence the value. Keep in mind, the dollar values given to posts and comment are in USD, but in fact the reward pool distributes Steem Power (Steem) and Steem Backed Dollars (SBD), both these currencies are tradable at Exchanges for BTC, and BTC is tradable for USD. All have their values and fluctuates. Your vote value may just be on the flip side of rounding, ie 0,49 ct becomes 0ct and 0,51ct becomes 1ct. Then the stuff I mentioned in previous comment, the vote power in your bucket (you can check that with steemnow.com).