You can see that here:
Showcase-GCC
That LINK takes me to Showcase INDEX page, not to a Category Page. The Showcase Index page is going to list all content in the system (just like the XenForo Resource Manager Index page lists all content in the system, just like the XenForo Media Gallery index page lists all content in the system).
This is a Category Page:
Members Ride's <--- look at the URL, it includes the route /categories/category-title.category_id
I get everything in my showcase
That is only possible on the Showcase INDEX page (and only when Showcase Home is configured to be a standard index).
A Category will only display content within its TREE.
Using your Category Tree setup, when you click on Members Rides, its going to display content from Members Ride's Category AND all of its children (Roadster, Coupe, GT40, Mustang, Other), which is what its currently doing.
As you can see here, we are viewing the Members Ride's category, which display content from the Members Ride's Category AND all of its children (Roadster, Coupe, GT40, Mustang, Other). There are only 3 items in the Members Ride's Category TREE (all in the Roadster category, which will display in its PARENT, which is Member Rides). If any of the other sub categories (Coupe, GT40, Mustang, Other) had items, those items would ALSO display in the Members Ride's category (as content from the ENTIRE TREE is displayed).
If you click on Roadster, its only going to display content in the Roadster category as the Roadster category does not have any sub categories. If the Category Roadster had any sub categories and any of those sub categories had content, those items would display in the Roadster Category AS WELL AS in the Members Ride's Category (as again, the entire TREE of content is display).
If you click on the Coupe category, its going to be an empty listing as it has no content posted in it yet. However, if you create an item in the Coupe Category, its going to display in the Coupe Category AND in the Members Rides, but it won't display in Roadster or GT40 or Mustang or Other as those categories are in a different BRANCH of the Tree.
Same thing applies to the Articles Category. When viewing the Articles Category, its going to display content from ALL of its children (Show Information, Track Day, Cruise, Build, Maintenance).
I've navigated through your Showcase and not a single CATEGORY LINK takes me to a Category Page that lists ALL content in the system. The only place that does that is Showcase Index page.
Maybe configure Showcase to use Modular Index instead of Standard Index. That way you can customize Showcase Home to look more like a Portal vs a listing of all content in the system?