One experience I had (coming from an Austrian right wing province) is that a significant share of polled people will not reveal to the pollster they are voting for the xenophobic candidate, because they don't want to be seen as a bigot.
It is like when your doctor is asking you if you eat fast food — some people will downplay it because they know it is wrong, but do it anyways in a "weak" moment when nobody is looking.
So suddenly in my village where I know everybody 56% voted for the right wing candidate, yet everybody¹ claimed not to do that when asked before or after.
¹: except one or two open Nazis
This has a name: https://en.wikipedia.org/wiki/Shy_Tory_factor
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This is the most likely explaination imo. But even then it should be possible to use bayes rule to price it into the result.
Even outside the polls. Trump rallies really started to empty out in the last weeks.