When a customer's cart is empty during checkout, it means the ecommerce system has not yet recognized them or recorded their cart contents. If the system recognizes them, it will display their cart contents.

It's common for ecommerce systems to have this persistent cart feature. It uses a cookie to identify the shopper, which means that to see the items in their cart, the shopper must click through on the same device and web browser they were using before the cookie expires.

Integration with ecommerce systems to pass the cart contents or a Cart ID through the URL is possible, allowing the cart to be rebuilt on any device.

However, this generally requires code customization on your ecommerce system to handle it.