I don’t think Exeter being in New Hampshire makes a difference for NMSF. Someone could correct me if I am worng, but I think all boarding schools have to meet the special higher boarding school NMSF index cutoff, so the NMSF data we have should be comparable among boarding schools. Day schools, on the other hand, will be subject to the state index cutoff of the state in which they are located.
That being said, the entire boarding school complex is quite a lot lower on the NMSF metric than the “best” day schools. It’s not even close. Only literally 10 or 12 boarding schools post numbers higher than you expect from a random group of PSAT takers, and only Exeter and Andover (roughly 60% of the Exeter number) are at all notable. I find the relatively low numbers of NMSF at bs versus day schools pretty interesting.