If you are really undecided...don't you think you are most likely going to stay home? When the majority of people do stay home...the undecideds by and large are the non-voters who get polled. It makes sense really. A lot of people are going to answer a poll saying they are registered and vote etc...makes them look like a better citizen to the pollster. In actuality they may have no intention of voting and say undecided to the pollster because they don't have a clue about those running and undecided is a safe response.
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.