Atheists and Agnostics Most Knowledgeable About Religion

I have long suspected this was true; weird but true...

The Pew Forum on Religion and Public Life released the results of their U.S. Religious Knowledge Survey:

"Atheists and agnostics, Jews and Mormons are among the highest-scoring groups on a new Pew Forum survey of religious knowledge, outperforming evangelical Protestants, mainline Protestants and Catholics on questions about the core teachings, history and leading figures of major world religions."

But why is it true?