Hi,
It could be that you configured HikaShop to allow the customers to pay for their order after the checkout, and even change the payment method. In that case, if the user doesn't complete the payment after the checkout and access his order details page, he can change his payment method. It's possible that the mechanism there allows for the selection of the normal PayPal Checkout payment method.
Supposing that this is how it happened, you should see the order being changed in the history of the order in your backend. That's actually the first thing I would check if I were you.