I don't see why this would cause people to vote differently from their beliefs on a referendum on the exact issue. This seems like a generic reason why people might vote for politicians who support things differently than how they are polled, but not responsive to what I am saying.

You're right, I misinterpreted what you were saying, and the explanation I described does not address that.

I think the explanation here is that referendum is anonymous, while polls not so much. When questioned by the person doing the poll people tend to give answers that they think are expected from them (i.e. Politically Correct), at referendum they are free to vote according to their actual beliefs.

If you have an opinion on an issue but don't care strongly about it, you might be willing to answer when polled, but unwilling to actively participate in a referendum.