Apple Pay in person, using a card I hadn’t authorised

This last week I attended a concert and went to buy a t-shirt at the end of it. I offered my iPhone with my Apple Wallet, choosing my Mettle bank account to pay.

My phone showed that the payment hadn’t gone through, but the vendor’s showed that it had. Confused, but reassured that I had paid for the t-shirt, I left.

This morning the transaction has appeared on my Nationwide bank account. I didn’t switch cards at the checkout, and I didn’t use Touch ID to authorise this second payment from the other account. So how did this happen?