Why have I received a credit to my account?
The AdWords Discounter reduces your actual cost-per-click (CPC) based on your ad's clickthrough rate (CTR). From January 14th to February 17th, 2005, we incorrectly predicted the CTR for a small number of ads created during this period; therefore, some ads didn't receive their full discount. We've corrected the problem and credited the price difference to affected advertisers' accounts. You can see this credit as a line item on your AdWords account Billing Summary page.
Thanks Google!