Google also knows how many Android users search/download the app. I don't use pinterest, but if they use Google analytics or ads then Google also has a fair amount of data on usage.
Google is very much a panopticon, I doubt any other single entity has as much knowledge of the internet as a whole. Now whether this reflects in Google analytics I don't know, as I've never really used their analytics for anything serious.
I believe this is just a broader result set that includes searches for anything that Google deems to be related to 'Pinterest the social network'.
Google doesn't have access to internal Pinterest metrics.