The more "value" a post has, the more time it will get to accumulate new up votes. So for example if you make a post with 50 Up votes, 1 hour left till payout and a "whale" up votes it, depending on the "whale's" Steem Power your post will gain more time to accumulate votes.

The estimated value of your post will also change due to the market price of Steem changing. It's based on the median of price the last 7 days - the witnesses publish the price feeds.

For the purpose of this answer, let's assume the question is:
**Why does my payout keep changing when I've received no new upvotes or flags?**

There is only a select amount of Steem to be dispersed in a rolling 24 hour period. At any given time, the system is making an estimate of how much you will be paid based on what percentage of Steem Power-adjusted votes have been given to your post.

Let's pretend a post is currently worth $5. But then, a bunch of other posts starts getting really popular. The money to pay those popular posts needs to come from somewhere, so a portion of your post's value is reallocated to these newly-popular posts. Now your post might only be worth $4.

Unless the dev team makes an update to change this (<a href="https://github.com/steemit/steemit.com/issues/87">Github issue 87</a>), the 25% drop in payout that happens right at the end should be explained.

One reason your payout changes is because of the price of Steem going up or down.