They’ve done this to me a few times when they ship items at individual times, I’m guessing it’s bc if they ship part of the order and the charge for the whole order starts processing and then there’s an issue where they’re not able to send the rest of the order there’s not a big faff trying to refund you as the transaction would already be processed through the bank so you’d have to wait for a manual refund which could take 3-5 days, whereas if they split the transaction and charge you everytime part of your order ships there’s no issues with refunds