This makes no sense to him, but she doesn't seem to be at at all alarmed. It's helpful for keeping his own mood relatively level, perhaps in a way he's not even sensing, himself. Accustomed to most people (aside from his son) tiring of him eventually, he is used to tones shifting, giving him some signal that it's time for him to be silent and disappear.

His answer is quiet, like he expects it will be what finally turns the conversation. "'m sorry. That doesn't make sense to me."