Interesting that doctoral degrees do not pay more on average than professional ones! Perhaps it is because most PhD's enter the field of research as opposed to industry? I haven't met that many PhD's in industry but there are a few.
It doesn't say "PhDs", it says "Doctoral degrees", which may mean (it isn't clear), that EdD (Doctor of Education, a common degree for public school administrators), DBA (Doctor of Business Administration, usually from Harvard), EDB (Executive Doctor of Business, typically offered to already successful business people) and other similar specialized but not necessarily "professional" degrees (which I interpret to mean MD, JD, DDS, chiropractic, etc.) would be in this group.
My undergrad university sends me begging letters - the place I did my Phd doesn't. They don't consider it worth the postage cost given what Phds earn!