This is because your eBay store has been configured to use "Custom Categories" for navigation.

Our system maps your product listings to eBay categories, but you can also create your own categories in your eBay store for your products.

If you wish to use your own custom categories, you will need to set these up in your eBay store and transfer your items to these categories within eBay.

Further information, can be found here on eBay.