Conway admits Trump is behind, is counting on undecideds

[Alcon]

Truman won this way in 1948, it's never over until every vote has been counted.

Correct.  And Trump is the one person who can inspire people who have never voted before and are not counted in polls.  People who like straight talk.  So far all he's done is win just about everywhere.  His primary victories in the northeast, where he won every single county, were historic.

There is not strong evidence that Trump has prompted a surge in voter registrations or participation by non-regular Presidential voters.  To the extent he drove turnout among new Primary voters, they were overwhelmingly already voting in Presidential Generals.  IIRC, the proportion of Trump voters who were new registrants was about the same as Clinton voters who were, and less than the proportion of Sanders voters who were.

Also, it's not like new registrants are never counted in polls.  Likely voter screens often ding them for that (rightfully so!), but depending on what other questions they ask (such as certainty to vote/"do you have a plan to vote?" stuff), they can pass a screen.  But, again, there is no evidence that Trump can safely depend on these voters, and a lot of them come from Clinton-leaning demographics anyway.

It's also worth noting, as Speed of Sound said above, Trump tended to slightly underperform his final polls in the primaries.