They are all given a clean slate somewhat and are exposed to the exact same environment

OK, try actually imagining this "clean slate" and "same environment". Imagine yourself, right now, going to boot camp alongside someone who's lived a life of relative privilege with lots of educational and exercise opportunities - someone better educated, fitter, even taller than you. Imagine yourself alongside someone who lived a life lacking in privileges - a crappy or unfinished education, not particularly healthy, shorter than you.
Imagine how you and those two hypothetical people tackle the different obstacles this "same environment" gives you - imagine going through an actual obstacle course with them. How easy or hard does each person find each obstacle? Will a short person find climbing a rope or wall as easy as a tall person? How about balancing on a log, crawling under wire, or jumping over a ditch?
A "clean slate" isn't any kind of reset of their abilities and experiences. "The same environment" inherently means it challenges their individual abilities differently. They get different outcomes because they were different people to begin with, so react differently to their experiences, like everywhere else in life.