The finished quest log is sorted by how you finish the quests themselves. The example you gave (Unbound) is a bit of a bad example because no matter what you will always complete that quest first. It is the gateway to the game and will always be completed before anything else in the game. Let's use With Friends Like These, The Pale Lady and Mourning Never Comes. When you complete the first Dark Brotherhood quest, it will show up as completed. Then if you complete The Pale Lady, it will show up after the aforementioned quest, even though Mourning Never Comes is the next quest for TDBH.
LikeASir189's post in Skyrim - How is the Quest Log Sorted? was marked as the answer