Hi @IvCHo, I see what you mean and find this:
Since Tips & Tricks and Tutorials is broken into 4 sub-categories, one needs to first be selected. They are in that "all" drop-down in your graphic.
Wouldn't be a bad idea to clarify this, but I'm not sure how it might be done.
EDIT: A little more...
I see that "all" is also not an option for Free Culture and Multilingual Discussion. A sub-category must be selected.
But Support & Help Requests, Development Discussion and Multimedia Showcase all allow it, I assume because those categories include both main and sub categories.