When an insurance company uses the word “adjustment”, they mean the discount they have “negotiated” (dictated) with your “provider” (doctor, pharmacy, hospital, nursing home…..). This is the amount of the charge that they are not going to pay. If the “provider” is “participating”, they cannot collect this money from you. If they do not participate, they can collect this money from you.
Generally speaking, the doctor had nothing to say about how much the discount was. If the group is large enough, or if they are the only group in town, they can cut down the discount a bit. If the insurance company is powerful enough (think Medicare), this is not possible.