Never mind mathematics, variance et all. It is time for some magic words...
*withholding attack?*
Found a block at 28% luck and crying about a withholding attack? *sigh* We've had far worse rounds in the past.
Obviously. Also, the insidious withholding attack was so well executed that it intercepted and nuked golden nonces from all the users of the pool for the 12 hours it was ongoing.