I believe that one factor is the number of vouchers issued which can’t be used online and require a phone call. BA implemented this to hoard cash, if the amounts were relatively small it was so much easier for the customer to use the web for a voucher rather than call for a refund. Then you come to use these and have to call. I remember reading somewhere the process for the agent was very time consuming and the average call length increased. This in turn led to the recent move to refund certain vouchers as the original cash and Avios, supposedly to ease the load on call centres.