The difference is that in a lot of cases, there are kind of a "hard cap" for demand that can't be increased easily without a lot of factor (accountant, lawyer comes to mind). However, I believe there are certain professions where if you're only concerned with the superstar level, the sky is the limit (demand-wise): there are no artificial (market) cap on the value you can added to the society.

That is until human is immortal, energy is unlimited (to a first approximation), and/ or the Singularity happened.