This is actually troubling. Law of averages say that random wild excursions from the mean without any other input (like someone turning off the NSA's rig ) should be very rare, and the larger the excursion, the more rare.

Im sure someone is doing some stats analysis to see if this the fluctuations are expected or a very rare anomaly - if they're happenign too frequently, then either ther'es very large flux in computation rate for a reason (NSA, or a major blackout somewhere, or internet outrage), or someone's not aggregating hashrates properly.

as you say, tracking the variance and assigning a statistical value to its probability would be useful. cept i failed stats class.