I don't quite buy it. Opinion polls should be a pretty risk-free way to talk about your real preferences. If people are afraid to expose their controversial views as a checkbox on an anonymous form, why aren't they afraid to do so in the voting booth too?
There is an expression "voting-booth candidate", meaning someone that people don't feel comfortable telling the pollsters they'll vote for, even when they will. Jesse Helms polled as losing his senate seat a couple of times when he won. And Marion Barry's comeback bid as mayor of Washington, DC, looked dead: he won by a solid margin.
A lot of political polls are phone-based, at least in the US. Maybe talking to someone, even someone you don't know and are not likely to meet again, triggers the same inhibitions that you have picked up via interactions in your social circle?
