What do they mean? 224 million "Christians" in the US. What do they believe, what beliefs do they share?
I know some people that identify themselves as "Christian" who don't go to church. They don't pray or read the Bible, and they really don't know any doctrines. They were brought up in a Church, or had exposure to Church of some sort, they live in a nation that is mainly Christian, and they have been surrounded by Christian iconography and culture their whole lives. If they lived in another nation with a different dominant religion, they would identify with that most likely.
I guess I am saying that from my time on the forums, most of the Christians on these forums would not consider the majority of the 224 million Americans that identify themselves as "Christian", as "real Christians".