If you try to delete a post/topic, you click on the "delete"-button at the post.
Then you check "delete permanently" and submit the page. Then you'll get redirected to the topic again and this post appears hidden, juzt like you'd deleted it "soft".
If you now click on this post to show it, you can click again on "delete" and this will remove it permanently.
But it should be deleted permanently when you choose the checkbox on the first attempt and get soft-deleted if you did not check this box.