I believe that the meaning of this has to do with *how* you delete a tree-mail, as you "move" it to the deleted option here
Which is also listed among all your other folders, giving the assumption that the deleted section must also be a folder... but it in fact, is not a folder and does not appear on the sidebar.
I'm assuming that there is no actual deleted folder and the tree-mail is instantly tossed out the window despite it seeming as if it's been moved rather than being banished to the shadow realm.