It can go up or down based on a few different things. One factor is the market price of Steem, though I think the internal number is based on a one-week average. Second, the quantity of Steem is always increasing, which dilutes everything. Be thankful you weren't here before the first July 4 payout; one of the first posts went down from $140,000 to $2,500 before the payout, mainly due to dilution (now, at least we have payouts every day). Also, if a whale (large Steem Power holder) votes on your post, but then goes off and votes on a bunch more in a short period, this can make each vote worth less. Those are the reasons, as I understand, that a post's value can change. It is always just an estimate until the actual payout.