Capitec loans

I settled my outstanding balance which was reflecting in my app and in bureau and my balance was 0.02 , but a week later capitec debited my account with an amount close to R400 saying is interest charges I don't know, they charge that interest based on what but what I believe is when you settle your debts before time is because you avoid being charge interest. Their consulted were unable to help even their complaints department they tell me it is because I paid it via Nedbank which to me it doesn't make sense a payment is a payment regardless how I paid it.