The data was crunched using something called a location quotient, which is used "to capture how common something is in a given location."
As Howmuch.net explains: "For example, petroleum engineers are relatively hard to find across the county, but you can find more than 50% of them in Texas. We mapped these so-called 'iconic' jobs by color-coding their popularity in each state and we added the average salary as a reference."
That explains why you'll find logging workers toiling away for $39,000 annually in Oregon, or why fashion designers are most concentrated in New York (or perhaps within several blocks of one another in Manhattan), with an average salary of $71,000. The map definitely throws some curveballs: For instance, Nebraska appears to be the preeminent place for meat, poultry and fish cutters and trimmers, while Hawaii's most predictable job is a dancer.