We're seeing a similar polling phenomenon in Florida and North Carolina. In both states, most of the polls are showing the race as a tie, with a one point or so advantage for either candidate. But then a small number of polls, again in both states, are showing Romney up by 5 or 6 points. But there are no polls showing a result in between. Strange.
This means that the right approach is to average out all polls showing NC and FL tied, then to separately average out all polls showing Romney lead and finally to make an average of the previous two averages.