After testing various parameters, I found a way to get 2mhash/s more out of diablo compared to phatk. Great!

However, for the last 5 hours, deepbit never calculated a hashrate for me that was even close to the 270mhash/s shown by diablo. It was mostly ~150mhash/s, sometimes as "high" as 200mhash/s. (Using a 45 min average) (also, 1.65% stale shares - might be caused by deepbit though, not sure if long polling works 100% again)
Is it possible that certain parameters cause diablo to display a much higher hashrate than actually achieved (for example because some of the calculations fail silently), or is this just very bad luck?
I am seeing this same effect on other pools. I am also wondering if Diablo's hashrate calculations are wrong.