Anecdotal, but when I was working maintenance in Florida I had a co-worker from P.R who was a welder before inhaling some nasty fumes and being forced to go on disability. He worked part-time with me doing maintenance; and I found out that despite knowing as much as I did and being older with more experience he was getting paid 2 dollars less than I was.

Basically being the only maintenance guys that place had, me and him went up to my [redacted] church nazi boss at the time and threatened to quit unless we "fixed the difference". She was ready to let us go until the priest there demanded she keep us because some bullshit "stations of the cross" holiday was coming up.

That was an example of racial bias in pay difference despite overwhelming qualities of the other person. Not particularly "unqualified" like you asked; but if this happens so commonly that I can just throw an anecdote at you...what makes you think that it doesn't happen on far worse scales across the entire country?