I guess I do not find that surprising because at least as far back as when I was applying, Duke had a sort of special appeal to certain applicants. Some of that was about sports, some about regionalism, and so on.
And I think that helped make it a bit harder admit at least in some cases than its academic peers which were otherwise quite different. And in other cases it was maybe an easier admit. But overall it does not surprise me a given school might send more kids to Dartmouth than Duke, or vice-versa, since they are really such different colleges.