Well when I say different, I really mean different. I know friends who litterally only hang out with people of a certain type: for example some of my asian friends, apart from me, only hang out with other people of asian descent.
Because I speak several languages, there are friends with whom I speak another language with. Of course, they could all speak English if they had to, but culturally they're totally different.
Some of my friends are gay/camp, whereas others are totally homophobic so clearly they wouldn't mix. Some are of the intellectual, a bit culturally snobbish, whereas some enjoy being more laddish.
Sure there are compatibilities among some of these friends but a lot of these friends seem happy to hang with people with the same mindset than them and don't really want to be with people with whom it's only going to be awkward.
For example, would most people invite their straight-edge mormon friend to a night clubbing in London? Or take their intellectual friend who goes to museums often for a piss up with the lads? Most people I know often say "No, I won't invite him/her because it's not their thing". The trouble is, I find myself saying that all the time.
Mainly because I never really feel like I fit in perfectly, it's emphasised even more in my circle of friends.