I am not jumping you at all. I am just saying that just to get in you have to pass certain apptitude tests both written and physically. Once you get into basic training, you are talking a different tune. If you cant keep up with the training, you will not complete the training. Basic training is HARD CORE training and it separates the men from the boys. They may lower the bar for meps, but once you get in you have to be able to hold your own.
To your point, my brother had a buddy who graduated high school with a MISERABLE gpa (like a 1.7 or something) then spent a few years in the Marines. Somehow, he managed to get into the University of Arkansas and dude graduated something like Suma Cum Laude and went on to med school at a pretty prestigious college.
Clearly the guy just needed some time to grow up and possibly just a good swift kick in the backside. That's the most extreme story I've heard, but you hear other stories where people weren't stupid (as far as IQ), in high school they just weren't motivated. The military gave them a chance to pull themselves together.