Google announced on the AdSense blog that they have now added deductions and their reasons to the AdSense payment reports. So instead of just showing estimated versus finalized earnings, Google will show you why your finalized earnings are less than your estimated.
Starting with May’s payment history, you’ll be able to see the invalid activity deductions that cause these differences. For example, if your estimated earnings were $1,100 and your finalized earnings were $1,000, you now have a better view into how your estimated earnings break down into invalid activity and finalized earnings.
Here is a picture of a sample payment report showing these deductions:
Last month, I saw a $20.13 deduction for "invalid traffic," who knew. :)
Forum discussion at Google+.