It's expected value is higher of what I get on slush's pool, and since the pool is meant to get your throughput (almost) at the expected value, it's either something wrong with me and the pool, or an over optimistic calculator.
Also, I went mining for about 5 days, with an expected 3 days of payout, but I found nothing. But that sounded perfectly normal to my ears.
1) My pool keeps 2%, not 10%
2) Mining is still probabilistic, even in the pool. Even pool have bad or good days.
3) There were some outages, because keep getwork based pool up is quite difficult. This also cut the daily reward
4) I don't know your hashrate, but score based system makes pool mining more probabilistic than share based. If you have poor hashrate, you still might be more unlucky.