Think of the simple phrase "Bill's friend".
If you were going to turn this around using the preposition 'of' would you say:
- A friend of Bill's or
- A friend of Bill
It appears to me that, in the US anyway, people always say "A friend of Bill's".
Even though I'm a native American English speaker, this just sounds weird to me. It seems to create a 'double possessive' (a term I just invented). I always want to respond: "A friend of Bill's what?" A friend of Bill's aunt?
So what's at work here, and do British English speakers also do this?