[Alberta's Child]

The “Doctoral Degree” category doesn’t just cover medical doctors. It includes a lot of people with a PhD in some arcane subject matter who work in research positions, or even outside their field of expertise entirely.

[FredZarguna]

I doubt it covers MD's at all. Most economists don't classify MD's or JD's as doctoral degrees. Neither do most business schools or stats. They're professional degrees [and note, their mean income is higher than PhD's, as shown in the chart.]