Your comparison is occuring during a period of bad luck. We've been going at 20-30% above average for this difficulty in the past 48 hours, and close to average prior to that. This is standard variance and can hit any pool including DeepBit (and has in the past). If you had tried about 2 weeks ago at BTC Guild we were averaging about 30% -under- the average for the difficulty.
Larger pools do not eliminate variance, it only helps smooth the effects of variance. The only relevant factors when comparing pools are: Fee %, Stale %, Downtime %. Those are the only 3 items that can be measured which have an effect on your payouts, everything else is chance.
In that case you may want to consider posting the pools "luck" stats in an easy to find place. Deepbit posts it + or -%.
I'll be making that a part of block statistics shortly. For the record, our luck has been the following:
Past 24 hours: 546k average per block = 25.5% bad luck
Past 48 hours: 587k average per block = 35.1% bad luck
Past 72 hours: 567k average per block = 30.4% bad luck
Past 96 hours: 527k average per block = 21.2% bad luck
Since current difficulty of 434882: 481k average per block = 10.6% bad luck
So if your time frame was the last 4 days, the payouts were 21.2% worse than the expected average for your machines. If it was the past 2 days, it was 35.1% worse. For comparison, our average shares at the last difficulty (244112) was 212437, which is 13% ahead of expectations. Meaning the average between this difficulty and last difficulty, the pool is still ahead by about 2.4% (this is overly simplified and wrong to just compare two luck percentages for rounds which vary in length, but the point behind it is still valid).
