Random sampling error, which is why you should look at averages of polls instead of individual. I would say they underestimated the national lead and overestimated the swing state. Also, what states did they consider swing states?
Edit: You are mistaken:
http://www.washingtonpost.com/blogs/...-swing-states/
Pulling out respondents in these eight states — all of which Obama won in 2008 — from the new national poll, shows Obama at 52 percent and Romney at 41 among likely voters. We report these numbers to help connect the dots between the clear Obama leads in the collection of states and the continued closer parity nationally, not to suggest pinpoint precision on what’s happening now in any particular state.Out of 929 registered voters in the new poll, 161 live in one of these eight states, with a margin of sampling error of eight points. The likely voter sample in these states is about nine points, making the 11-point gap an apparent edge.



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