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