AdSenseAdvisor - 7:41 pm on Mar 16, 2011 (gmt 0)

The variation you noticed is part of normal and ongoing testing we perform to improve our ad formats. They only run for a very short amount of time and are only shown to a very small percentage of your users.

Our engineers are always closely monitoring the results of our test, and any variations should impact only a small percentage of your impressions.

Thanks!
ASA.