Just to be fair to Google, they can split the ad revenue anyway they choose. If one reads last quarter’s income statement and others, Google reports that the cost of acquiring revenue keeps dropping. So, maybe Google is a more efficient organization and operating leaner (they should do this BTW), or they are paying less for employees (?) or they are paying out less monies for their revenue share partners (joint venture organization, AdSense publishers, and others) – you pick.
Also to be fair to the AdSense lovers here there is a basket full of other reasons why your revenue is dropping as well. Yes, it could be your fault as some will soon suggest.
I suspect many web sites AdSense revenues are dropping because of ALL of the reasons posted on this and the other 600 threads on this subject.
Unfortunately, you will not know what has happened to your AdSense revenue until Google opens the box and lets you see the details. NEVER!