I did notice one interesting thing in the survey results which no one else commented on:

While "Atheists/Agnostics" ranked high in their knowledge of religion, people who said their religion was "nothing in particular" ranked rather low.

One wonders, what is the difference between "atheists/agnostics" and "nothing in particular?" Well, my first guess is, the "atheists/agnostics" are people who are interested in religion (as a social phenomenon, or whatever), and the "nothing in particular" people who are not interested, and therefore don't even bother to adopt a label for themselves. Another possibility: the "nothing in particular" people believe in some kind of god, but they don't want to join any religious group. But that doesn't explain why they would rank so low in their knowlege of religion--or does it? I guess we are free to speculate.