Take a forum with (at least) these three categories and topic counts:
You might then see this in the category filter dropdown:
x 4 here is referring to just the topics in the
Announcements category but not in its children categories.
I think the purpose for these
x # labels must be to give users a sense of how many topics they’ll see when they filter by that category. But when you filter by a parent category, you also see the children categories in the resulting topic list.
I think it would be better to show the count of topics in the category and all sub-categories in this dropdown.