Very true. It's entirely possible that people who save up their own money in order to make an investment in themselves through education are simply more shrewd by enough that even if they went another route, they'd make $6k more than the median.
I know we interviewed a CC grads (more out of kindness/curiosity than anything else) and while the guy wasn't what we were looking for, he definitely put a lot more effort into it than most people his age likely would have.
There is a high likelihood of causation though, simply because many jobs (especially in the healthcare industry) require an associate's degree. For many people earning those degree help them go from minimum wage jobs to 30-40K per year.
People who work straight out of HS are very different from those who decided to save up money and go to college, even before they go to college.