A questionable undervote would be a ballot where, say, there are dimples next to two presidential candidates. An undervote because the machine didn't detect any punchout. As I said, I don't know what they're getting at in the film.
The overvotes _weren't_ being counted in some places, iirc, so it's possible that the writers in the film meant that if _only_ the undervotes were counted, not the overvotes, Bush would've won. But these are speculations on my part as to the intent of the film-makers. What I'm sure about is that the Florida media consortium found that under all rational statewide recounts of contested ballots, Gore won, but not by much.
Here are a couple of Robert Parry articles on it:
http://www.consortiumnews.com/2001/111201a.html http://www.consortiumnews.com/2001/120601a.html
A scurrilous mainstream treatment: http://www.cbsnews.com/stories/2001/11/12/politics/main317662.shtml
Most papers ran a story November 12, 2001.
By the way, Yoshie, the filmmakers wanted to call the movie an "An American Coup" but were convinced that they would never be able to get it on US TV with such a title. They still weren't, as far as I know, so there might be a lesson there.
Jenny Brown