It can work the other way around too...

Let's say a post is currently worth $5. Over the next few days, new Steem is created, but only half as many people are voting compared to recent days. That means there's a lot of new Steem with nowhere to go, so it goes to the posts that already has votes - the ones with pending payouts. Now your $5 post may have increased to $6.

This isn't exactly how it works, but it's the simplest way I know how to explain it.