I think the simplest explanation is that pollsters are from the media/elite; when Trumpers get a phone call from a pollster, it's either their chance to "own the libs" by lying, or they're embarrassed to admit their reactionary beliefs so they say what they think a centrist or liberal would say. Then they vote for the furthest right wing loonies they can find.

You can see this kind of thing in practice if you've ever been a non-white person in a redneck bar.