That's a reasonable answer for why there's error, but the question is not "why were your polls wrong", the question is "why did you trust your model despite overwhelming evidence that few public polls, nor most poll aggregators, nor the betting public, were making the same assumptions?" That's the real dirt that I hope comes out in campaign books. Was somebody deliberately releasing only the most optimistic projections? Or was there just a bubble that prevented those assumptions from being questioned?
(FWIW, Nate Silver analyzed the last three weeks of polling and found a R-ward bias in almost all of them, which may suggest Obama's numbers were improving in the last week or so of the campaign. But he says the bias is "probably more than can be explained by the late shift alone.")