Per trader_4:

It's also possible that the Trump vote count is under reported in polls
because people don't want to admit they will vote for him because in the
media world out there that makes them a racist, red neck, etc.

My son-in-law spotted that early on. His term is "Closet Trump
Supporters".

Also, my understanding is that polls have become problematic to the
extent that they are conducted via telephone and therefore only with
people who have land lines and who talk to telephone solicitors.
--
Pete Cresswell