Back to the original subject.
By your own logic, doesn't this imply that advanced is not pro.
The descending order implies pro is the top class.
Everything below pro is... not pro.
Most people who go into the advanced category are excellent players. Many are on the cusp of turning pro. Plus, we all figure out very quickly that it is just short for advanced amateur. If we replace 'Am' with 'Int' you could make the same argument and it would be more confusing. The abbreviation convention seems to fit, from my perspective. Do you think someone actually thought about this in the previous 35+ years?
Or should we change it to Professional, Amateur, Novice?