Google announced that they may now auto-pick the font size and face depending on the viewer's screen.
Google said they will "optimize your ad units for your unique site and users." That means, if you selected to use the "default" font option, Google will select the "best-performing option" automatically for you. The "best-performing option" may differ based on the type of user visiting your site, so to determine that, Google looks at operating system and browser and the ad unit itself (language, size, number of ads, etc).
Google said this only will work if you have your fonts set to “AdSense default font family” and “AdSense default font size” selected.
Back in July, we reported users noticing Google testing font size changes with their ads - I guess it was a test for this.
Forum discussion at WebmasterWorld.