WHY IS BUSH'S LEAD SMALLER IN THE THREE-WAY RACE?

If you follow the RealClearPolitics poll-of-polls average, the 3-way spread always seems to be smaller than the 2-way spread. It seems Kerry is better off with Nader in the race. Why?

My theory: when Nader is included in the question, the anybody-but-Nader voters switch to Kerry.