Is the statistic that "75% of prisoners are Christians" a joke?

I highly doubt it. Considering that most people are religious, it makes sense that most of the ppl in jail are religious. And to add insult to injury, I've met many christians that are mean, spiteful, hypocritical, rude, and/or trifflin... but I have yet to meet a non-believer who was just as bad or worse.