The categories page gives an inaccurate representation of the activity within a category / forum.
Within the screenshot above you will see
category stats (circled on right), these stats do not include any of the posts from the subcategories giving a negative appearance of inactivity with the forum / category.
In addition if there are no posts in the top-level category there are no numbers indicated in the topic stats - again reflecting negatively on the forum overall - because other categories do contain these stats.
A user reviewing a forum may check the categories section to see how active a specific category is in considering if they should join.
Personally I believe it should include the subcategories in the stats as visually the sub-categories are not seperate in the row the numbers are for. Either that or a change in display / representation of the subcategories.