It is because your phone app (the special card) is authenticated to Tesla. Once that is set up, and you are logged in. Any number of cards can be added. once a card is added, that can be used authenticate any fobs and other cards, you might have. When I took delivery, they only wanted to make sure the Tesla app on my phone was working. I set up all the cards and fobs at home, well after delivery.Not sure why they allow key card pairing without a pin or a special card. At this point, any card lets you add a new card. But at least it does warn you if the car and phone are online at the time.
Back then (June, 2022), the car was delivered with two cards, and two fobs, none of which were programmed. Your phone and your cards, support multiple vehicles. The fob only supports one car at a time.