I think it's the betwixt and between dynamic: working class folk know they're living on what they have coming; upper class folk know they're living on what they have; but middle class folk, no matter how they live, are only middle class folk if other middle class folk agree they are — hence the insecurity, and at one reason for the conformity.
(in the UK, I think U vs non-U started as a joke, yet was popularised by exactly the people it had been meant to be taking the piss from? https://en.wikipedia.org/wiki/U_and_non-U_English )