Suffficent people change their minds in elections, for the results to be in doubt. However during many years working in local party politics and marking up electrol rolls. What you do find is that many people always vote and many people never do. So most effort is put into the actual voters and to the new names on the roll. The party tellers role out side polling stations is important for this reason alone. As you have no idea how they voted what ever they may indicate. Who votes, always comes out of much the same pool. How they vote does change. Even if their declared party idenntity stays the same.