One possible scenario is that the surgery took longer than expected. Breast Augmentation is usually quoted for a typical amount of surgical time. In my surgery center--it is quoted based on 2 hours of surgery. However, a case can sometimes go longer than the average. While this is unusual, it does happen. If the surgery is much longer than the average estimated time, some Facilities will balance bill for the extra time in the OR. However, all of this should be stated in the Quotation that you received prior to surgery.
Alternatively, it could be a mistake, or perhaps the anesthesia bill was not part of the original fee quotation. You should call the office and speak with the person who quoted the procedure.