I've edited the breakdown.
What you need to do is click on edit then, if it's a maths article, say, select "physics" let it reset the menus, then click again and select "mathematics." You will then have all the categories available.
This is probably a bug, but at least there's a short term fix!
I had been trying that cristo but no luck so hopefully Greg has fixed it. I'll give it a go for the zeroth law entry.
EDIT: nope still nothing. Must just be mentors that can edit that.