I think Google just released some official announcement. Finally. According to its known issues page:
June 22, 2009: The amount displayed when you generate a 'Since last payment' report is currently incorrect, but our engineers are working to resolve the issue as soon as possible. Please be assured that this is only a display issue, and that you're being properly credited for your earnings.
Work-around: In the meantime, you can still calculate how much you've generated since your last payment. Generate an Advanced Report for your earnings from the current month and then add the total unpaid balance listed on your Payment History page. Thanks for your patience as we work to resolve this issue.
You can check this link:
https://www.google.com/adsense/suppo...n&answer=92365
Check these threads too (look for the posts by AdsensePro Obair and AdsensePro Jen):
AdSensePro Initiated: Drop in Reported Earnings - AdSense Help