Since I added "-v -w 128 -f0" I've been getting ~620 mhps. I've also noticed times when deepbit reports that I'm getting >700 mhps. I haven't logged onto the mining machine to see if poclbm-gui reports the same, so I don't know if deepbit is misreporting my mhps.
Deepbit can't see your exact mining speed, those numbers are guesstimated from number of submitted shares.You can be more lucky in one minute and less lucky in another, but overall it's pretty close to real value.
Sometimes you may find a run of many shares in a few seconds - then the number on luck meter will be higher.
You can open advanced settings page and increase averaging window from 7 minutes to 30 minutes and this reported speed will be more precise. But since it depends on luck, it's not guaranted to be 100% accurate.
On the other hand, your reward depends on the speed reported by "luck meter", and not the real hashrate, because the number of submitted shares is what you get your money for :)
Looking at numbers in your history column i can say that your actual speed is ~622 MH/s, because those are calculated for all the block's lenght and thus more accurate.
EDIT: and you forgot to erase your login name in the upper right corner :)
