Strange that the only medical specialties they pulled out were psychiatrist and surgeon. As you can guess from the gap between the two, there are a lot of specialties filling that space. Psychiatrists are among the lowest paid non-primary-care physicians.
“My neighbor the pilot put it this way…Pilot salaries are kind of like doctor salaries. Residents (new hires at commuter lines) work long hours for poor pay hoping to get the experience to move up to good salaries (like the young doctors in your group practice). The analogy breaks down because while medical residents choose their path early, pilots with a major airline can always move up with enough effort from Internal Medicine to Plastic Surgery”
Theoretically he’s correct. That would work if it was always a case of people just needing to get the experience, and then they locked into the high paying jobs. However, airline pilots are highly unionized, and every time you go to another airline, you start at the very bottom of the seniority list. And when your company lays you off, sometimes you never get recalled. Go elsewhere, and begin again at the bottom. I’ve flown with people who have been furloughed 4-5 times. Your carrier goes bankrupt? Tough luck, start at the bottom elsewhere. Some AA pilots were furloughed after 9/11 and just got recalled—about 13-14 years later. It is not always an upwards trajectory, and effort can mean far less than luck.
The two plastic surgeons I know make most of their “play” money by owning their “aesthetics center” in which they offer facials, spa treatments, sell stacks of beauty products unique to their brand, etc. Even with regular patients coming in for injectibles, it’s more lucrative to have regular patients whom their staffs can see for cents on the dollar. Their centers look like spas which happen to have a plastic surgeon on staff. 
And one of them does huge volume on men’s hair-loss replacement and drives a Bentley.
In one word: entrepreneurship. That’s hard to quantify in these types of surveys and many occupations do not lead easily to that.