Being Charged Different Subs for my Student Sons

Could someone please explain why my two sons, both over 21, both varsity students, who are both FULLY reliant on their father for all expenses, INCLUDING medical aid subscriptions, are being charged different amounts per month on KeyCare Plus - the Student Plan. The elder son (***) has the proper subscription based on the lowest income category, while the younger son (***) is being charged in a higher category, due to his bank statement showing income. This income is pocket money from his father for petrol and clothes. I have submitted an affidavit from him stating all this, including what the deposits in his bank account are, as well as proof that he is still a student. But still Discovery continue to tell the consultant at my place of work that they will not consider lowering the subscription of the youngest son. PLEASE could someone explain as this has been on going since mid January 2015.