The problem with the "undecided", is the assumption they are a static group. Like the poor, people move from wealth to poverty and from poverty to wealth. People's opinions change over time. At any point, the undecides will include some truly clueless people and some people whose opinions are changing. So, attempting to create a rule about there ultimate vote is foolish because. One might be able to make assumptions based on trends and events that have just occured but to create a rule out of past elections and apply them isn't going to work.
The polls have to be fairly close, but remember they are designed to make news for the most part and not give us a truly accurate picture.