my grip about this study is it seems to confuse the term atheist with non-religious. just because you haven't aligned yourself with one of the other relgions of the world does not make you an atheist.
an atheist believes that there is no god.
a non-religious (in the way they are using it here) means you aren't associated with any church/whatever.
as far as i'm concerned, atheisism is a religion. if you haven't given serious thought to your choice to not believe in god you are more agnostic then athiest imo.