The collective hashrate seems to bounce up and down randomly.
For example it was at 500ghash/s for hours, then suddenly, it dropped to 410. Too big of a swing happening to everyone at once to be pure variance IMO.
I checked my total hash rate and it went down about 20% for no reason
Edit: It happened again, all miners suddenly went down 50%, not a coincidence because total pool rate also halved
The stats on the web page don't mean much, they only show approximate values. I think it's because sometimes the pool website takes a 50 second long snapshot and sometimes a 70 second one, but calls them all mhash per minute. Stats are also randomly delayed, so that having good stats available can't be used against the pool. If you are mining and sending in accepted shares, then you get paid accurately for your work.