A lot if Immigration Officers, especially at hubs, know full well what a MR is but play dumb.

Last year a friend and I did a MR into ORD from LHR. We were in line for the same officer and my friend went ahead on his own. When I followed after he had cleared, I got the "what's a mileage run" routine. After I cleared, my friend told me had spent 5 minutes explaining it and had been told by the officer that he saw it every day!