The brain generates a characteristic signal (from a sub-region of Broca’s area) when it detects grammatical errors—but it generates an identical signal when you’re listening to a grammatical sentence and need to re-parse it partway through. I think this latter case is actually the real purpose of the signal: every time it triggers, your brain is warning you that you need to stop and check the sentence again even if the meaning seems unambiguous. So the “pretending they can’t understand you” reaction could just be a reflexive response to that signal (i.e., the brain is telling them it’s confused even if there’s no logical reason it should be).