it's because the pending payout is an estimation based on the current price of steem in external markets. So if steem price goes down pending payout is soon to follow, if it goes up the opposite happens, even without number of votes changing, but in either case the amount of steem you gain from the post remains the same from the 50/50 split.
Thanks! I mean like this comment of your it payout total of $0.19 now, then it depleted tomorrow. I noticed it last time i checked.