After introducing 'Standard AdSense font family' back in February, Google AdSense is updating font styles for its ad units once again of late. Reportedly, AdSense is changing the default font used in the different ad units and it's done on the basis of which ad font performs best in a particular ad unit.
Here are the new defaults:
Arial: 728×90, 336×280, 120×600, 120×240
Verdana: 300×250, 160×600, 468×60, 250×250, 234×60, 125×125, 180×150
Times New Roman: 200×200
The new fonts are expected to come into play within the next few days!