The displayed hashrate is only rough approximate. The formula is based on the count of your shares and some constants. payments do not depend on it! your payouts are calculated on the real sent shares.
I have tuned this formula, now you should see nearest hashrate.
Thanks for the quick response.
I wasn't so much concerned with payments, as just having a good reference to diagnose my hardware against.
I do see an improvement, but I think now you're slightly over-reporting. I have a R9 290 that has constantly been reported by cgminer as 453 for days on end now, and yet DwarfPool now reports it as 480.