The effect, some have suggested, makes the top content-column ads look more like the organic search results below, in that they stretch out further to the right.
Google isn’t commenting beyond its usual response to tests seen in the wild. “We are always experimenting with the look and feel of our search result pages, including the delivery of relevant advertising,” spokesperson Jim Prosser told me when I inquired about the widely-seen test.
The company continually tries different displays in an effort to boost click-through rates. It recently changed the way it shows the domain portion of the URL in its AdWords ads, so that the domain is consistently lower case. In announcing that change, the company said it helped improve many of its metrics, including click-through rate.