On 23 Nov, 21:07, wrote:
Why does the system need to know more than which /card/ is being topped up?

Because you need to be able to administer auto top-up, in particular
if a top-up fails* they email you to say your Oyster Card will be
blacklisted if you don't sort it out. Plus adding and removing cards
and so on, plus I'm not sure banking rules would allow you to have a
debit card on file for someone without having their personal details.

(* Credit is added to your Oyster card before they do the bank
transaction to verify you have the money)

U