OK, I THINK I just about get it. Basically the very irregular manner in which the hashes are accepted by each individual worker, plus the fact that as you say the average is only taken over 5 / 10 mins means those guestimates are pretty much worthless to everyone.
Actually I think the replies to your question were misleading. You were right to ask. What you seemed to be saying was that your hashrate on your miner was reporting at about 2400KH/s, and yet even
when the pool shows approximately 2400KH/s for the recent shares period (about 10 minutes), then that estimate is wrong. You weren't saying that your actual DRK/day differs from the pool estimate when the pool is reporting wildly different hashrates, but when it's actually about right.
In that case, the pool is simply wrong. Lots of pools have completely incorrect calculations for the estimates.