Two factors came into play.
Caucus locations kept changing, so there's no telling how many people (regular voters, not rabid supporters) didn't get notified of the new locations.
Plus, at the last minute, the GOP decided that the precincts could allow unregistered voters to vote if they signed an affidavit. Some precincts allowed it and others didn't.
We probably have a smaller turnout due to the incorrect information given on caucus locations and Mitt probably got a fair share of the last-minute affidavit voters.