It should be noted that the atheist sample is fairly small, and that the majority of people who identify as atheists as adults were not raised as atheists. CARA goes into detail as follows:
Some of the retention rates in the figure above were never provided in Pew's original report. These are calculated from the original data sets released by Pew for this study (one for the continental U.S. and another for Alaska and Hawaii). In these data there are 432 weighted respondents who say they were raised as Atheists. A total of 131 of these individuals self-identify as an Atheist at the time of the survey resulting in an estimated retention rate of 30%. However, there were a total of 1,387 Atheists (weighted) identified in the survey (equivalent to 1.6% of the adult population). What these findings reflect is that in the U.S. Atheists are for the most part "made" as adults after being raised in another faith. It appears to be much more challenging to raise one's child as an Atheist and have them maintain this identity in their life. Of those raised as Atheists, 30% are now affiliated with a Protestant denomination, 10% are Catholic, 2% are Jewish, 1% are Mormon, and 1% are Pagan.