Lots of web site do a reverse "DNS" look-up on your IP address, by doing this they know where your IP address comes from. I live in Royston, which is about 20 miles south of Cambridge but my ISP, Virgin, route everything through there large Cambridge hub, hence everyone thinks I am based there.
Before anybody relies, I am not looking for a hot date in Cambridge tonight.... which is the usual use of this type of advert :-)
David