Ordinary Banana

is there a pithy law & economics term for “the law as various observers believe it to be” vs what it actually is in practice?

The premise of your question is fatally flawed. See Robert Ellickson, Order without Law. The law *is* what it is believed to be.

I’m asking about how the law & economics model treats this, is that what you’re answering?

Yes

so what’s the word for fake law?

What is on the books is not very often consulted.

so law is what people believe but there’s no term for the distinction between that and “on the books” law?

Well there are rule of law issues that stem from that distinction. But the point of the economic analysis done by Ellickson and others is that the distinction doesn't matter in many, if not most cases in which people have to decide how to behave or even how to resolve disputes.