Why does Google show URL's in some formats and not others?

[crick]

I have noticed they don't show it in the horizontal ad blocks but show it in the vertical ones. It would be good if they don't show it all. This would stop users from typing the URL's when they come across an ad they are interested in. I know this does not happen much but the more adsense aware people become the more this is likely to happen.

[FourDegreez]

I wondered the same thing.

[hunderdown]

Personally, I prefer the ones that have the URLs--I think it gives some added credibility. I think that's more of a plus than the minus of people taking the time to type URLs.

I would guess they've done some A/B testing and found that certain formats perform best with URLs, and others without.