I think it most likely doesn't include the 'don't know, don't care' crowd. So technically those folks would be not atheists now.
I'm trying to drill down to the actual study report but haven't been able to get to it so far (the pages keep becoming unresponsive on my phone for some reason). I'm curious about their methodology.
One thing that occurs to me: atheists only make up a small percentage of the population, and atheists who raise their children to be atheists only make up a small percentage of atheists. If their sample size of "people who were raised by their parents to be atheists" is small, then we would expect quite a bit of error in their results. Without knowing how many people they surveyed in each group, it's hard to say how reliable the study results are.
Also, I can't help noticing two things:
- lots of Catholic commentator (including one bishop I found) gloating over how the study indicates that Catholics do better than most other Christian denominations and that the low retention of atheists shows that people have an "innate longing for God".
- the survey was done by a Catholic research organization (the Centre for Applied Research in the Apostolate).
... so the results do appear to be self-serving. Were they manipulated to be that way? I don't know - I know nothing about CARA's reputation and haven't been able to look into this study in detail. But the incentive is certainly there.