Neither the underline nor the nessie button is visible on 335x280 or 300x250 or 300x600 ads. So it is just the large title (without the underline) which acts as the clickable area on the ads. Is this the primary reason for the drop in CTR? I am not sure why the nessie button disappeared too!
or is it that they are experimenting this (no nessie and no title underline) on a few sites ?