Few remarks:

1. How the variance works in reality can be observed at antpool stats. With their 200+ PH/s in the pool their results have significant statistical relevance. For example their daily luck chart here https://www.antpool.com/poolStats.htm shows that daily results oscilate between 80% and 120% almost every 5th day. These 20% oscillations are purely variance based and can be interpolated to whole network and can easy answer those hundreds petahashes jumps in global daily hashrate calculations.

2. Theoretical background to variance is nicely described here, but is not that easy to read:
http://organofcorti.blogspot.sk/2015/07/faq-bitcoin-mining-and-luck-statistic.html