Why are Catholics singled out from "Christians"?

I've noticed that some people would single out "Catholics" from "Christians". Belief.net is one website that separates "Catholicism" from "Christianity", and the website does not claim to be affiliated with any religious organization. Sometimes, a public university's student health center may classify "Catholic" and "Christian" into two separate categories under "Religion". Assuming that everyone has benign intentions, why are Catholics singled out from "Christians"?