Put another way, a 20% difficulty increase - which we just had - means, if BTC price remains the same, that it'll take 20% longer to get a block, and hence your income just dropped by 20%.
M
I was just looking it over and the rate at which the difficulty and hashrate is increasing at an alarming rate. M
When I joined this pool feb '16, difficulty was 163.5G. It took 11 months to double to 336.9G in Jan '17. It more than doubled again after 6 months in Jun of '17 at 678.6G. Now it's more than doubled again in 4 months to 1452.8G. Fortunately BTC price has more than kept pace. If I remember right BTC was between $200 - $400 then.
It disturbs me that the cycle of the hashrate doubling keeps shortening, without the release of a new miner. I know more players have entered the miner market, but they all use the same foundries. Are the foundries getting more efficient at producing and just putting out chips that much more quickly, or has some player come up with a more efficient way to use the existing 16nm chips and just not released it to the public yet?
I think hashrate and difficulty keep increasing because the price of BTC keeps climbing and climbing and climbing. That causes more miners to enter the game with the existing hardware.
It's an equilibrium that I'm not sure all miners understand.
"Oh, look, I can make this much $$$ by adding this much hash power!"
"Wait, why did the difficulty go up 20%?"
"And my $$$ just went down by 20%?!"
"Maybe I should add some more hash power..."
M
(Which is super easy when your hashrate is as low as mine.) 
Keep it up and YOU'RE going to jinx us! 
