Even if, and I doubt that if, the usual pollsters are trying to be accurate, the greater majority of Trump’s support is not in any of their databases of usual suspects to poll.
The polls, as events have shown throughout Trumps primary, are only able to approach the appearance of accuracy if the contest is limited only to the usual DNC vs GOPe matches.
A large percentage of Trump’s support comes from folks who’ve otherwise given up on the process of elections since they were never represented.
If Trump voters don’t tell the pollster they are voting for Trump, the Dems won’t know how many votes to steal.