Because many private insurance plans (as well as Medicare advantage plans) have fee schedules that vary according to practice and specialty such that, in some instances, political favor is given to primary care in the form of higher reimbursements, in other settings size translates into power where the large multi specialty group - including their mid levels and primary docs - enjoy a more advantageous fee schedule, etc.
...and yes, varies from state to state and entity to entity.
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