Hopsital billing inaccuracies are not uncommon. Sometimes the patients are overcharged but just as often patients are undercharged. Several studies on the cost of inpatient versus outpatient operations have pointed this problem out.
Hospital charges for materials used don't always match actual materials used. Patients can be double billed by mistake or not billed at all.
It's always wise to review medical bills. Mistakes happen and should be reported. When too many errors occur, the hospital or doctor will be alerted to the need to review billing practices.