Not that it isn't probably still terrible code but the banking system is likely logging the full transaction details every time it is modified / passed to a different part of the system. I.e. fully auditable - if something goes wrong with the transaction it can be fully traced back, corrected and the underlying issue investigated.
One large bank I used to work for had a payment processing system doing about 40 thousand transactions per day. It ran on about $8 million worth of hardware - mainly IBM mainframe. I was impressed until I found out each transaction was essentially a 60-100 byte text message. I can confidently say a well designed system can do 1000 times that load on an iPhone.