It's a dynamic free market thing, there is not fixed value. Supply and demand.
A lot has to do with time and circumstance as well. How anxious a company is to get a slot filled can raise the price, how anxious the applicant is to get the job can lower it. Negotiation skills, as well as willingness to take a risk and walk from an offer have an effect.
Stability is another factor. One may look for a bigger pay from a job that may not last more than 2 or 3 years. One may accept less pay from a company whose employees tend to stay for decades.
Self reporting surveys are also a problem. People being paid well are more likely to fill out these surveys (or even exaggerate) than people who are unsatisfied with their position.
[Self reporting, frequently used for nutrition studies, is highly suspect. Recently researchers gathered together self reported dietary information from many studies and analyzed the results. Apparently about 2/3 of the people in those studies should already be dead from malnutrition.]
-- FORTRAN manual for Xerox Computers --