I think I pinned down the distinction here.

If you tell something like this: "yes, I'm pissed that you shat in the bed again because I was about to go on break and now I can't and I'm hungry and cranky.", the patient is going to form a lot of important beliefs regarding the question they're asked that are not true, more than if you say "this doesn't bother me". You have to say what ever sentence ends up misleading the patient the least about what they want to know.

For the affair on the other hand, it is not so, they'd form more valid beliefs if you said that you are having an affair, than if you say you don't.

The truth is such word noises, body language, intonation, and so on, that mislead the listener the least. Usually has to be approximate due to imperfect knowledge and so on.