The book reviews seem to indicate the author focused on rare and exceptional conversion and deconversion cases, and limited the study to 18-22 year olds at Canadian universities, an impressionable age where peer pressure remains very strong
I think the usual case tends to occur for someone who is slightly religious becoming more religious, or slightly religious becoming more atheist - and for intangible reasons, rather than through tortuous soul searching. My brother was fairly ambivalent about religion, until he started regularly attending Methodist service in his 40s.
The exceptional cases undoubtedly make for more interesting reading through.