You are the one sneering. "Failing to understand" is a completely respectful stance. It places the lack of understanding on the person saying it. It leaves open

You are the one sneering. "Failing to understand" is a completely respectful stance. It places the lack of understanding on the person saying it. It leaves open the possibility that the person saying it is wrong. It's skeptical but not without a degree of humility. This is opposed to a straight up judgement, like what you have just done.