jw's and Mormon's make more atheist than any other groups combined
It may be true. Unlike 99% of 'people of faith' many JWs and Mormons had to fearlessly apply facts and reason in order to escape a controlling cult. If they go on to apply those same critical thinking skills to all the claims of theism it will lead to atheism.
The vast majority of believers have never faced up to the failings of their religion because it doesn't demand much from them.
Of course many ex-JWs lose their nerve and stop at some sort of alternative faith system.