Yeah, you find that lots of people are friends with people you know, but wouldn't imagine to have any connections. But it happens in real life too.
A couple of years ago I went to a friend's bday party and played pool with one of his other friends who I thought I hadn't met before. A few minutes after the guy left I realised he was someone who I went to school with from primary school all the way til college. Not really that surprising when you consider all the variables, though.