That’s just how the default category boxes work right now. We could probably change the default behavior. Though I wonder what’s the intended experience in our documentation categories. It seems the index was added with the category banner in mind, rather than the category box:
At the same time the index topic stays pinned to the top of the topic list, it doesn’t automatically un-pin when the index topic is read. So we’re duplicating this index quite a bit: In the first place it’s meant to generate the sidebar menu. Then we pin it to the top of the topic list. Then we add a link to it to the visible category description.
I think we could actually drop the link from the description in the documentation categories.