it used to be some consistence, i used to predict my next month revenue just by counting my website growth rate. And my prediction were close enough.
Now i can predict #*$!
The entire system is a lot more complicated than it used to be, starting with many many many AdWords changes. That's not fishy, that's evolution. And unless you understand AdWords pretty closely, you'll never understand how AdSense ebbs and flows. You don't have the same ads, the same placements, the same type of bidding, the same devices and the same device bids now that you even had six months ago, let alone two or three years ago.
If you think it's "fishy" then I repeat - why run AdSense? I wouldn't allow ads on my site if I thought there was a better than average chance I was being ripped off. Why do you?