Not to undermine Military Cop’s posting, but I have an additional theory, based entirely on gut, with no proof whatsoever: The Pollsters were paid by DNC to show Romney ahead so he wouldn’t campaign harder, and would campaign in states where he was likely to loose.
Sun Tzu taught to encourage your opponent waste his efforts and to mislead him to thinking he has the upper hand, then let him waste his energies for nothing.
The GOP internal polls had to count for something. I believe Romney was confident because of what his own polling units were showing.
You have a valid point and, it fits.