If am correct, these were the pre-paid debit cards. These work differently than the regular credit / debit cards. In this particular incident, customers didnot loose money but instead banks did. I know this since the some members of the gang were based out of New York.
Usually, in the era of outsourcing as well, all the account details of a credit card customer including customer's PII (personally identifiable information) max withdrawal limit, account status etc etc is stored within the bank's network infrastructure. If at all any part of it is outsourced to any third party, then it would be just card manufacturing (but within a high security and cctv'ed location) and customer support services.
However, it is not the case in the pre-paid cards (atleast until this attack). Since pre-paid cards were seen as a low margin secondary market to credit cards, banks wanted to be in the market of pre-paid cards but the overhead to maintain that offering was more than their profits. So they outsourced everything to the companies like electracard. (Like how SBI cards has outsourced it to GE in India)
Now all the account details of a pre-paid card customer including customer's PII (personally identifiable information) max withdrawal limit, account status etc was stored on the servers of the outsourced company.
Since it is far more difficult to get into a bank network than the network of companies like these, the hackers took advantage of this loophole and what they did was.
They was a team operating from cayman islands or puerto rico or some island where extradition to US is not possible.
They had already hacked into electracard long ago and had a backdoor access.
The team in New York and other parts of USA brought a few hundred pre-paid cards, the hacking team went and increased the withdrawal limit of those cards into a very high number allowing the guys in USA to have literally unlimited withdrawal.
Then these guys changed locations and kept withdrawing the money from ATM's, shopped for things like ARMANI, GUCCI and SAKS products in New York's costly 5th Avenue (Just so that you don't know 5th Ave, here handbags and other accessories for women start from around ~$4,000.00 USD) and these products were then sold on international ebay by others or exported to other countries.
This attack showed everyone a loophole in outsourcing. AFAIK, none of the customer's account / cards were compromised or misused. The only entity that lost money was the banks (or their insurance companies if they had them insured). And am sure, VISA / MasterCard will impose fine on these banks as this process had to be checked as per PCI guidelines (Payment Card Industry) which the banks didn't.
Just my 2 satoshi's