Many of the polls also reflect the "wishes" of people who will vote for Romney later to be somebody else. Those who really want Santorum will say that they won't vote for Romney in the polls in the hopes that it will cause others to vote against him in the primary.
During a primary with 4 candidates, many people who support 1 over the rest will try to sway others by saying they'll never vote for so-and-so during a poll, but later when they are the candidate they not just support them, but more "loudly" than others.