First National Bank
Won't accept surname change

I changed my surname and after receiving my new id document from Home Affairs when to the bank to change my details and get a new bank card. There I was told that the system doesn't allow a surname change unless I produce a divorce certificate. So can I now asume that FNB is a higher authority than the Department of Home Affairs? Now I have to go through life with two surnames as they don't accept my legal one?