Supporters of other candidates felt that a long race could possibly benefit them, mainly Edwards and Richardson. They feared that if Obama won New Hampshire it would be over (similar to Richardson's supporters giving Obama Iowa). Edwards and Richardson both fell a few points short of what they were polling, and Hillary was the one that these points went to.
The median average of polls for the 6th and 7th of January was as follows: Obama 38%; Clinton 29%; Edwards 19%; Richardson 6%; Others 3%.
The actual result was as follows: Obama 36%; Clinton 39%; Edwards 17%; Richardson 5%; Others 3%.
The difference in polling and results for Clinton's and Obama's competitors is clearly not enough to account for such a difference overall.
Well, the reason I got a different spread was because I discounted some of the polls. For example, the Zogby (always an awful poll) which had Obama up 13% and the Uni polls that came out. Rasmussen for example had the polling at 37-30-19-8, when the actual result was 36-39-17-5
Which comes out to:
Obama -1
Clinton +9
Edwards -2
Richardson -3
To mean, that means undecideds broke late to Clinton hard, as did supporters of other candidates. As you can see, Obama got what he polled.
