I don't wish to debate the pro's and con's of atheism, just help me understand your personal
reasonings for this huge difference in figures.
Without reading any of the other responses...
Most of the population have not been involved in cult-like religions. Most people go about their daily lives, and pretend to be holy once a week, month or year. They claim to have a particular faith or to believe in God, but never really live their lives that much different than those who don't claim to be believers. And they're never called on it. They can fake it, go to church once in awhile, give some $ to the church every so often and all is good.
People who have escaped religions like the JWs and Mormons, or even more cultish, give more thought into religion and the existence of God. Since these people, who lived years, decades, lifetimes, trying to live up to an unattainable standard, when they realize the falseness of it, they question all of it. And when they question it to a much deeper degree than the average person, they come up with the only reasonable answer that works...that either there is no God or he doesn't exist as the churches would have us believe.
It's just another case of the average American believing what people in authority tell them without really questioning it.