The Great Myth of the "Undecided..."
Almost all of them have a strong "lean" tendency, whether consciously or unconsciously, towards one candidate or another, and come voting day, they will be significantly more than likely cast their vote towards that "lean."
There aren't 10% out there who are truly "undecided" and up for grabs. There's a spectrum of "leans" who, for whatever reasons or reservations, don't want to commit their position to a stranger on the phone.
The thing I'm trying to say is that not all undecideds are practically "available" to both candidates.
Polling is a blunt instrument.