I wouldn't say it's _completely_ separate, in the sense that within a certain field + industry combination, the salaries will be fairly stable, and from there it's a combination of skill, worth to company, being able to market yourself, be visible, and being in a position (financially and otherwise) to be able to bargain effectively for your salary. Along with your networks, looks, etc. With an added heap of some randomness.

All of these add a lot of noise, but there's still an underlying signal there. It's kind of like BMI; at an individual level, it may be highly misleading due to the variance, but at a population level, it's fairly insightful.