Well it's looking like they assumed more people were going to show up, based on their polling, and a good chunk didn't show up. That is, Trump voters showed up approximately as the poll models predicted, while Hillary voters apparently weren't as committed to actually showing up as they were when polled. So the polling fouled up only in terms of capturing a meaningful nuance whether the voter was likely to show up, which is a really hard thing to do.

And the polls didn't ask, and may have a lot of error capturing for, the Q&A along the lines of "if there's another controversy regarding candidate X are you a lot/a little more or less likely to vote?" It's plausible the polls don't capture last minute antipathy.