21% of Atheists Believe in God

According to a new poll of the U.S. religious landscape by the Pew Forum on Religion & Public Life, 21% of atheists don't understand the basic concept of atheism:
If you believe in God, doesn't that mean you're not an atheist? I have a feeling what we have here is a group of people who think it's politically correct among their friends and associates to declare themselves an atheist, but down deep really believe there's a God.

Either that or they're far more confused than even I thought.