Most banks use an eventually consistent message oriented architecture, not a transactional one.
At a low level banks use both absolute consistency (for physical transaction stores) and eventual consistency (for logical transaction implementations). The logical transaction implementation abstracts inter-bank and physical payment messaging.
It would be impossible to have absolute consistency in the logical transaction layer as transaction scopes would have to be open (i.e. locked) for days at a time in some cases. That simply doesn't scale.
Ultimately banks have millions if not billions of pounds floating around not in traditional transactional stores all the time.
I agree that ATM's are a bad solution to this. The issue there though is loose coupling of third party financial networks though compared to a general need for consistency in terms of one's own financial needs.
But loose coupling between third party payment networks (say debit card purchases over Cirrus) and the bank is not really the same problem as using Cassandra at Facebook.
At a low level banks use both absolute consistency (for physical transaction stores) and eventual consistency (for logical transaction implementations). The logical transaction implementation abstracts inter-bank and physical payment messaging.
It would be impossible to have absolute consistency in the logical transaction layer as transaction scopes would have to be open (i.e. locked) for days at a time in some cases. That simply doesn't scale.
Ultimately banks have millions if not billions of pounds floating around not in traditional transactional stores all the time.