The Truth About Credit Card Expiry

I used to be dumb enough to give my credit card number to Sony PSN. (No more! From now on I will be using gift cards for online services like that)

I had a MasterCard registered with Sony. I bought some PSN credit in January.
My card changed in Feb. (same card number (PAN), new expiry and CVC numbers)
I had never given Sony my new expiry # so I thought I was OK despite the hacking. After all, when I got the new card the activation instructions said that once you activate the new card, the old one becomes useless and should be destroyed.

I logged into PSN on the weekend to double check what card they had and it was that card.
I started a transaction for $5 worth of credit to see if it would show me for sure the expiry date was the old one... I clicked next and got back a thank you. A minute later an e-mail receipt arrived showing I had just purchased $5 of wallet credit in PSN. WHAT!?!!?

How was Sony able to process a purchase without the new expiry?

I got on the phone with the credit card company and asked them. They told me that because the old card has not yet expired, even though the new one is activated, they still keep the old one active too, and that is why they ask you to cut up the card when you have activated the new one. (funny that's not how they explain the process in the letter that comes with the new card)

So I reported my card lost and had a new one with all new numbers issued.