It's because you are mining in a relatively small pool.
Small pools = big payments, but more seldom
Big pools = small payments, but more often
If you want to be paid most often, go with the biggest pool.
If you want the most money over time, you have to look at many more factors. For instance smaller pools often cause fewer stale shares, which may result in higher pay over time. They may have other advantages as well.
Exactly, you get paid on if and when that pool finds a block.