The traditional explanation for this phenomenon is that the subset of campaign polls that are released to the public is subject to a type of selection bias. Campaigns conduct polls all the time, but only occasionally disclose these results to the public and will be much more inclined to do so when the numbers are favorable for their candidates (especially in comparison to independent polls). In essence, the internal polls that filter their way into the public domain may be the outliers.
This is certainly an important part of the story, but my view is that it lets the campaigns off a little too easily.Plus, Nate hits Politico here.
Greg Mitchell is the author of more than a dozen books (see right rail of this blog). His latest, on the Obama-Romney battle, is "Tricks, Lies, and Videotape."