When you buy something with 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 for the returned amount. The card issuer then posts the credit to your account.