Ask HN: Why were the US election polls so far off?
For pretty much the entire election season the polls saw Clinton ahead of Trump - with still as much as 48% to 44% on election day. What was the problem? Selection of focus groups? Sample sizes?
See also Obama's last election - if the error margin votes had gone the other way he'd have narrowly lost.
EDIT: Also, "Shy voter" is probably a thing. This campaign, and a lot of recent US politics, is deeply polarised. It's not surprising people don't say they were going to vote for trump when asked.