It's called genetics. The majority of those inmates are black americans who have very good genetics for sports, weightlight and powerlifting.
I've met several people in my gym who'd put some of best bodybuilders on this forum(no disrespect meant) to shame, and most of them aren't even serious and work bluecollar jobs with very long hours.
Plus, Blockhead made an excellent point. These guys are in a constant state of rest and are very methodical, so they're bound to get good results anyways.