Most people are not willing to inform anyone who they are really voting for. They are tired of being labeled as homophobic, Islamaphobic, misogynistic, and racist. So they just keep their mouths shut and tell people what they want to hear and then do what they want when they actually vote.
That is why Hilary was going to win in a landslide that did not happen. The only people that believe the MSM polls is the MSM. It is going to be a repeat of 16 in 18, and in 20.
Remember, margin of error works both ways. If a poll shows Trump ahead by 48 to 45 and the margin of error is three points...then that means Trump's support could be as low as 45 or as high as 51. Likewise, the dem could be as low as 42 or as high as 48.