It's defined by the federal government by region and classification. The classifications are pretty formalized as part of the visa process.

My point was the data is wrong. It classifies someone doing php work for a small non-profit as the same as someone developer high-frequency trading systems for an investment bank, even though in reality there can be an order of magnitude difference in salaries.

The whole concept of "prevailing wage" is a sham in high-skill occupations. It's a hangover from blue-collar jobs.