they probably feel that it is unnecessary to delve into the details of any other religion when they've already found what they believe.
This
it would appear that once people choose a religion many don't even bother to become enlightened of its teachings.
That's the odd part. I think it's skewed based on level of religious dedication. I dearly hope that anyone participating in Sunday morning church, Wednesday night church, and a weekly small group session would be able to score highly on their own religion, even the first quote means they bomb tests on other religions. But for those who claim to be religious without actually following the respective religion (likely most people), they would get the worst of both tests.
As for the high scores of atheists, I can somewhat understand that, too. As someone else mentioned, there may be a goal (whether good or bad) for atheists to learn more about religions. In college, I was a dick of an atheist. I read through the Skeptics Annotated Bible and countless religious arguments just to better argue my belief against street preachers and friends. It took a long time for me to realize how much of a jerk I was being, but in the meantime, I still studied religions.
And where are Nehemiah and the other vocal religious players? We need more opinions.