Google announced that they may now auto-Select the font size and face depending on the viewer's screen.
Google says they will "Optimize Your ad units for Your unique site and the user." That means, if You choose to use the "default" font option, Google will choose "the best performing" option automatically for You. "Selection of the best performing" may vary based on the type of users who visit your site, so as to determine that Google looks at the operating system and browser and ad unit itself (language, size, number of ads, etc.).
Google says this will only work if you have a default font font family "AdSense" and "AdSense default font size" is selected.
Back in July, we reported that Google takes into account the user font size changes with testing their ad-I think it is a test for this.
If working correctly it can be a great feature. But all those who work with Google