Chemist claimed by using member instead of depende

On the 20th of July, I consulted at Dr. Kotze at Reyno Ridge and was given a prescription to go to a chemist. I chose to go to the one directly across from the doctor as it was nearby. After getting my medication I went home. A few weeks later GEMS, our medical aid fund sent my mother a bill saying that she has exceeded her amount for medication. Then we checked the date and it was the day I bought the medication. My mother and i share the same first initial, I suppose this is where the misunderstanding came from. Then GEMS advised us that we have 4 months to reverse this claim to my name, but the chemist refused saying we should have come the day before. Why didn't they ask? Why do they keep on assuming?