I'd not thought about it before, to be fair, but reflecting on it now - I could imagine that within a family you get ideas & preconceptions about other family members that people from outside the family might not be prone to. So Ginty and Ann could just have expectations not only about who is who depending on context, but also 'Nick does/is like this - Lawrie does/is like that', and an outsider might be less likely to fall into those traps? Not that that explains Miranda.