If they believe in God, they aren't atheists. If a man tells you he's German, but he has a Japanese birth certificate, passport, nothing but Japanese ancestors for the past 30 generations, and is monolingual in Japanese, do you believe him when he says he's German? No, you think "this guy is a liar."

So, at best, these atheists are most likely just self-described and are really just Christians trying to "get back" at God.

Plus: this study does not describe its methodology at all, so, I'd take its results with more than a few grains of salt.