Atheists don't believe in religion. If you put yourself in the non-religious/secular/non-practicing category, you are an atheist. Our society attaches such a stigma to that word that many, many people are just afraid to admit it, or even think about it too deeply.

If you force people to choose between religious or atheist, a large percentage will choose religious. When you add non-religious or secular as a theoretical middle ground, the numbers fall from the religious category.