Pre Authorisation means nothing

I recently went for in room procedure, and app**** for a pre-authorisation (pre-auth # ***0) beforehand. The pre-auhtorisation was received and I confirmed with Discovery (*** FAZLOEN) that everything will be paid for as on the Quote received from the doctor. I then went ahad and paid for the procedure thinking I would be refunded. <br> <br> After the procedure, come the day I had to claim Discovery refused to pay as they said there was a problem with the ICD10 code... If this is the case how did discovery pre-authorise the procedure as the ICD10 code was valid as on the quote. I spent days and multiple calls trying trying to get information out of Discovery, but I was made empty promises on the phone and had to start afresh describing what happened every time I called. Eventually I reverted back to the doctor to sort out the matter with Discovery, and they paid eventually, but unfortunately it was short by about R4500.00. My question remains why pre-authorise and tell the customer that everything will be covered in full, when it won't.