There are several possible reasons for this difference. Sometimes it depends on the coding used to describe your condition. You may both have back pain but you may not have the same diagnosis. Cost and insurance coverage can vary depending on the problem being treated.
You insurance coverage may also determine what is billed and what is paid. Deductibles, copayments, and coinsurance can all make a difference in the final billing.
Costs can also vary based on how much time the patient is with the doctor. Extra tests such as X-rays, MRIs, or CT scans may add to the total bill for one patient but not another.
To know for sure, you'll need to talk to the billing staff in the doctor's office. Don't hesitate to call and check it out.