A month ago, Google launched native A/B testing for AdSense ads.
When Google was 95% confident of an ad type being a winner, it would show you the results and let you choose the winner to continue to run.
The thing is, although it does show you the results, the buttons to select the winner to run are grayed out.
A WebmasterWorld thread has one publisher who wrote, "when I view it [the winning ad], the "pick a winner" buttons are disabled and the is no sufficient data showing."
Others have reported the same issue.
The work around, just edited the ad unit manually to change it to the winning format.
Forum discussion at WebmasterWorld.