The value of the rewards shown on steemit.com and other frontends depends on the market price of the STEEM token. It uses a 3.5 day average of the price. So the reason you saw a lower reward value is because the 3.5 day average price of STEEM was lower when you looked a day later than when you looked originally. Similarly if the market price of STEEM increases the payout value you see will increase.