Well, I think, for better or worse, it does take a pretty big commitment following a long thought process to call oneself an atheist, and I'm not sure most prisoners are capable of that.
Perhaps a more interesting question would be, how many people in jail who consider themselves Christians fit the image according to Christians themselves? (Or sub any other religion).
It's just that I think there is a very broad base for being a Christian (I consider myself one - and I'm completely and totally different from many on here, as they'll attest), but there's really not such a broad base for being an atheist.
That in itself would probably skew the data.