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.htmlHi thanks for the comments and links, will take a look later. I have this feeling that more of what we see is just variance based than people give it credit for. I am just blundering around here a bit, but have never let lack of knowledge or experience get in way of trying things and gaining some first hand experience.
Rich