polls are used to tell us that A is winning. well, who wants to spend 3 hours voting for a loser (B), so you give up. you don't vote, and A wins.
well, the rats are catching on to this trick, and we're not believing the polls, so we're going to spend our time and vote for B.
and we're not going to tell you, the pollster, that we are going to do so. so there.