I'm guessing they're only looking at the Referrer field, and they're doing it across the board. (For me, Google's result pages all point back to Google, so they

I'm guessing they're only looking at the Referrer field, and they're doing it across the board. (For me, Google's result pages all point back to Google, so they can leak this info via the Referrer field.) I'd also guess that the data from domain specific sites are more valuable than generic search sites. (User selects appropriate site, does search, selects appropriate result.)