A bit of ambiguity about certain things is an important lubricant for almost all non-romantic relationships. My family has a good idea of my salary (as pointed out elsewhere: they can see my house and my car) but I keep the specific number to myself. They're left with just a vague notion rather than anything they could do math with. It's much the same as how they only have a vague notion about what I do in the bathroom. They can use their imagination if they'd like and infer it from various things. But I shut the door so the obvious isn't rendered so plain to see as to become a topic of awkward (and sometimes destructive) conversation.