The reason why no one wants to touch this question is that based upon what you've described you're entitled to prevailing wages.
So, the only thing that anyone can reasonably advise is for you to contact the U.S. Department of Labor and request an investigation (866-487-9243). The alternative would be for a lawyer demand an explanation on your behalf from your employer, and if the explanation is not forthcoming, then file suit and let the employer explain the rationale for your exception to the court. That could cost you about $5,000 easy, assuming that your employer is correct on the law. So, it's not a good risk, and most attorneys won't want to try to completely analyze your circumstance to try to divine the precise reason why your employer thinks it's paying you lawfully.
Anyway, I've seen your question sitting here for way too long, so I thought I'd respond and give you my 2 cents worth.
Hope this helps.
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