The reason for this is that you have not created a Menu Item for the new article "Portree Campsite". So that "Portree Campsite" article is associated to the parent "Camp Sites" menu item.

That list shows the menu items, not the articles. You need to create a menu item for the menu items list to update 😜
If old articles are still there, it means that you have not deleted the menu item for it.

If you want the module to only show in the Category page, I would suggest to not use the Menu Item assignment, but instead, use the Joomla Content assignment.

Select the Category that you want, and under it, select only the "Categories" checkbox (uncheck the "Articles" checkbox).

Haha, no problem 🙂