When you buy something with your Wirex card, the merchant requests payment from the card issuer, not from you directly. The card issuer pays the merchant and adds the purchase amount to your account balance. You then pay the card issuer.
Returning a purchase works the same way, but in reverse. When a retailer issues a refund, the money doesn't go directly to you. (This is why most merchants won't give you a cash refund for a purchase made with a credit card.) Instead, they ask your card issuer to credit your account with the returned amount. The card issuer then adds the credit to your account.