In all likelihood, if he wins, it's because something major happens in the next 12 days that we can't account for yet.
If the election were held today and he won, it truly means the polls were straight up wrong. (Unlike 2016, when his victory seemed unlikely but was in a reasonable margin of error).
Or perhaps that Democrats racked up an unlikely combo of landslide victories in safe states and TONS of near-misses in swing states, so that Biden wins the popular vote by a huge margin but Trump ekes out a stunningly close victory in like 10 states.
The prospect of that last scenario genuinely gave me butterflies in my stomach just now. The good news, though, is the state polls show Biden comfortably ahead you're more than enough states to reach 270. He would have to actively lose support during the next week and a half for that doomsday scenario to occur.
God does the Electoral College suck.