Google Maps on Android is the same. Using my home Wifi, it gets the location to less than a meter or two. i.e. it pinpoints the specific area of the phone socket my ADSL connects to. Of course that must be a fluke since I could move my ADSL to one of the other sockets and it wouldn't know any better.

I had presumed that at some time possibly when the GPS and Wifi were both on at home, it matched the external IP against the GPS location?