I guess the number of topics in a category should be equal to the sum of topics in all its subcategories. but in the hamburger menu, it isn’t:
the strange minus topic number happened at some time point where we added some sub-categories to the category and moved all topics from the main category to its’ subsequent sub-category.
This is not the case on the other category selector either.
Though I can’t find anything from the search I’m pretty sure this has been discussed a few times (or more) and it would have been changed if they had no good reason to keep it this way.
That said I don’t really like this either as it can be somewhat confusing.
For example, I have an archive category which has 0 message:
This is confusing because it tells me at first glance that there is nothing to see in it. But it’s wrong!
If we click it anyway, we can notice that it is not empty and has some subcategories with messages: