On the mobile category page, there is a button to view all the categories in the topic with text like:
view all n…
Bug is that n refers to the number of topics in the category excluding subcategories, while the button navigates you to the category topic list including subcategories.
I think the navigation is right, but the number should be changed to reflect where is sends you. (you could also argue for the removal of the button, which would be fine with me since you can navigate with the category badges).