Because that's when you will get rewards. If you have rewards ready to be cashed out sooner, they will show up. Or at least that's what I wanted to code. It shows the "distance" in days from today until the next payment.

Later edit: I'll look into your blog to see when you have to receive rewards and see if there's any inconsistency. It might be related to the fact that I'm not getting more than 40 posts in total, in my query. Looks like you found a bug (or at least a use case that I didn't take into account).