This is really about the so-called Bradley effect. It is interesting to talk about but, as I recall, it was pretty much disproven in the 2008 presidential election.
Because he was largely an unknown, masquerading as a moderate. Things have changed, most definitely, but there's still the residual effect of the outing of "Joe the Plumber" and the fear of being quoted as anti-Obama in a local news source. Ordinary folks fulfilling their civic duty at the polls don't need that hassle. They have jobs and families to protect.
In the case of the telephone polls, you may be right. Most people I know don't answer anonymous calls flagged by Caller ID, so there's probably an element of the least informed and most susceptible to major media cheerleading in that data set.