I'm of the opinion that this is exactly what's happening.
Imagine how tempting it must be when you can increase your profits (and thus inflate your share price) by a few hundred million by tweaking a single line of code. Of course, that can't go on for ever, and it it were ever proved, AdSense (and Google as a whole) would be dead in the water.
I believe this is the real reason Google do not tell publishers or advertisers what percentage they keep for themselves. They can change it whenever their profits need a quick boost. Unfortunately, it may also be their Achilles' heel.
Google's profits and share price are pretty much arbitrary and are not a genuine reflection on how the company is performing - which in the long term is bad news for the whole industry.