It takes some time for you to get full credit for your hash rate, since the pool looks at the last 5 ND shares when it calculates a payout.

If you start today, and have 10,000 shares, and we found a block, it looks back at the last 10 trillion shares (5 times the network difficulty.) - so of those shares, you only have a few percent, not your full hash rate.

As you mine for longer, your reward gets up to your actual hash rate, and more of those last 5ND shares are yours.

It all balances out, because if you stop mining, your shares from the past are still getting credit, even though they're dropping off.