what is the point of a Gap cover

I have been with Kaelo for over 14 years with no claim. I joined them because I was promised that they would cover for shortfalls that my medical aid doesn't pay. My son recently had an accident and was admitted to the hospital and needed surgery. He was operated on, and everything went well until I received a statement from the doctor that the medical aid only covered 2k from the 8k the doctor claimed, and I needed to cover the rest. I was not worried because I know I have the gap cover. I immediately contacted our medical aid consultant, who then sent me forms to fill and advised me what documents i needed to send to Kaelo, which I did. Now yesterday I received a rejection letter from them saying they won't pay because my medical aid did cover the procedure. My question is, what is the point of a GAP cover? Is it not to cover shortfalls? I feel like I have been throwing away my money for the past 14 years. I am not happy at all , i need a better explanation. Even the statement they sent appears as if the medical aid did not cover anything, whereas it partially paid the doctor.