Because there are intermediary bank fees and receiving bank fees for international money transfers. If you do not pay these fees in advance when you send the money, it will result in us receiving less money than you sent. If this happens, you do not have to re-pay the difference. We will add the difference to your next payment to save on bank fees. Conversely there are cases where you may have overpaid and we will also register this in your account and deduct the overpayment from your next payment.