It seems to me that this is the case: people who have been in the industry longer probably never had a degree, as it was much more common to drop out of college in the '80s and early '90s to pursue a game career. (Nowadays, people tend to complete their degrees and then go into game development.)

Since the people who have more experience get paid more, and the people who have more experience dropped out of college, it looks like the people who dropped out of college get paid more. But they didn't control for things like experience. So there you have it.