The system is PPLNS. Hence it's pay per last N shares. If you have a very low hashrate (speculating here, don't know your hashrate), then it may wildly fluctuate how many shares you have in the N. The easiest to compute is look at your transactions. Compute your hashpower / pool hashpower a bit before the block was found initially (look at the user stats) and multiply that by block reward (17.5Y right now). Does the number you get in transaction look about right?

An even more accurate way is look at the unpaid shares / pool shares. This gives a better estimate because it takes into account your actual shares that made through. For example:

Your Valid: 341080
Pool Valid: 927582

This means that I should be getting 36.7% of all YAC or 6.42Y / round. Payments have been ca 6.2..6.7 in the past hours so that seems to be about right. It's always somewhat a question of luck and how many shares you get it. And the total payout per day also depends how well the pool does. If it's a bit unlucky you get less, if it's a bit lucky you get more. Have you checked from transactions which blocks have been paid out, do you have an actual full days worth of blocks paid out or only some of them? The 520 confirmations is a bitch...