Here is the problem with trying to pin-down beliefs. We are constantly taking in new information and evaluating our beliefs. Our beliefs change over time. Society, the way we were brought up, a need for self-identity and identifying with a group, and the list goes on, all influence our beliefs. Our beliefs/lack of beliefs may not even be that strong, we've just never questioned them.
How many active JWs only remain so for sake of family, and not for belief? How many identify as Catholic, Jewish, Protestant, Hindu, etc. based on culture and not on belief? How many atheists claim to belong to no religion, yet have a wide variety of beliefs ranging from spirit-worlds, ghosts, to supernatural phenomena and re-incarnation?
I think an interesting statistic to show would be who is practicing the religion and personally ask the person what they believe, not who they identify with. You might get an interesting answer.
Why don't born-in atheists (for lack of a better term) remain atheists? That would be something to ask them. Perhaps they can't identify with other atheists, given atheists can have such a wide variety of beliefs in the after-life and the here-and-now. There isn't anything particularly unifying about being an atheist, unless you were born in a certain religion, and then proceeded to reject it (obviously like a lot of us here). You will most likely identify with that particular set of individuals, because you would share a common experience.