How are your miners configured? If it's Proportional in deepbit then there is a bit of luck involved: have you noticed the percentages in the Stats section? The second one, Average in the last 24 hours, seems to directly affect your 24h profits. If it's a negative percent then you can expect to get a little more than the calculated estimate for your current hashing speed. For example, my single 5870 at 447mh/s is expected to make 0.26 BTC/24h. Over the past couple days the stat on deepbit has been between 0.22 BTC and 0.31 BTC per 24h. I have used BTCGuild but I prefer deepbit for the chance of getting a better payout.