I doubt it's because of research. You mean researching the Bible, church history, etc. would cause someone to stop believing in God? I don't buy this in part because they continue to read and debate the Bible even after their conversion. And why would an atheist need to defend himself or herself?
For many of them, I tend to think it's because they suffered some tragedy, hardship, tough time, etc. and believe God abandoned them.
I'm not anywhere near being an atheist, however:
I'm sure a certain amount has because of a hardship or what ever.
To make a good argument against the existence of God you would need to know why a person should believe in God and how the religious beliefs systems work and the basis (scriptures, dogma etc...) those systems work from. That's why they would know more.
Also most people stop researching and exploring once they become "complete believers". I started asking my Pastor friend (years before he became a pastor) about some inconsistencies in certain sermons, and in the bible and he didn't have an answer for other than: "Take it up with God". Which meant even in the face of continuity discrepancies he refused to address them or ask questions about it. Which is OK, I'm criticizing him for it. He is truly a person who lives a true Christian life to the letter which for that i have the up most respect for even though i don't believe the same way.
For me i stopped believing in organized religion because i was at one time very enthusiastic about Christianity and kept learning about it. The more i learned the more i soon realized how much man really perverted it to a greater or lesser degree,IMO, the word of God in every text and source available today. I saw man's greed for control and power woven into the words, concepts, guidance, and philosophies of God.
The real question we should ask, is it really true most atheists know more about the Bible then true believers?