The additional payment is the difference between the original tariff and the new one or a charge that has to be paid. The refund is for the taxes that you paid for the original ticket and which will not be used for the new ticket or for taxes that are lower in the new combination. For technical reasons these taxes have to be shown separately.
 
For example: A customer originally booked a flight from Zurich-London City-Zurich. The flight was then changed to Zurich-London Heathrow-Zurich. The airport taxes at London Heathrow are a little lower than those at London City, so there is a part refund. If the tariff conditions contain a charge for rebooking, then a tax refund and a charge for fees will appear on the credit card statement.