I was young (no approach to statistic) , and, one of my first times in casino, I decided to try Martingale betting system using color doubling. All was working well, until I decided to double the minimum play and unluck suddenly hit me with same colour for 14 rounds in row . Those unrelated events definitively killed all my cash 
That was the first time I lie face to face to the concept you explained.

It's so easy for miners to use difficulty for income's calculations, but luck in BTC plays big role too and it's usually understimated when calculations are made.
I'm not sure that big mining operations think about the luck/unluck, but as margins are eroded the weight of variance increase.

For the long time there is nothing to worry about ; we are mining on a super optimized, honest project, and this is what make the difference  !!!

Good work Kano !