There is a thread from March 28, 2009 at AdSense help titled "Why did Google suddenly start taking 90% of my Total Earnings?!". Apparently a number of AdSense accounts are or were having a problem in getting credit for earnings.

google.com/support/forum/p/AdS ...

It's worth noting that the post is about YouTube partners.

"They are from AdSense overlay ads on my YouTube videos."
"Yesterday the Host earnings were 15% of Total earnings. Normally it is 75 – 80%."