It is the expectations game. Politics are about perception as much or more than about reality. And Trump may be a victim of his own bragging. He has been talking about all the poll having him up 20+ points, etc., so the expectation is that he will win by YUUUGE margins. So if he wins but wins by less than 10 points, for example, then the perception is that he lost a lot of support in a short period of time, and that will be the story heading into SC.
That is one of the reasons Rubio got such a bounce out of Iowa - no one was predicting him to do so well. And Cruz could be set to get a bounce out of NH - no one expects him to do well, so if he manages to come in 2nd or 3rd, he could use that as an indicator of momentum.