How does Fill with 5-man work?

I'm very curious about this, because often when me and my team 5-man a game, we all put fill, but we somehow always get the exact same role every time. Is there a reason for this? Like it doesn't matter in what order we joined the match, or anything else -- we just somehow always get the same role. Not that anyone complains, but we do it to switch things up. Not really switching things up when getting same role o.o;

Thanks!