About the perceived paradox between atheists having a low retention rate but are still growing: these are USA statistics.
Atheist are only a very very small % of the population. This group grows quickly because of all the believers turning atheist, not by forcing their kids to not believe.
Besides, there is a difference between being raised an atheist ('here are arguments to not believe in any gods') vs not being raised as a believer ('...'). The last group might be easy prey for religious smooth talk.
And... guess what happens if you were raised without getting an aversion against religion, and you find a nice significant other with strong religious feelings who asks you to join their church? (Which is a likely scenario as almost whole population is religious).
Thank God I live in Europe where much more people are unaffiliated and/or atheist :-)