I have decided to give a clue for #5: Notice earlier I pointed out that all of the items were in quests. That is important. Anything special about those quests? Reminder: I have already said it isn't to do with the amount of items required for the quest.
because the other items' quests all have a "usable" effect while the Sorbex Exterminator quest does not give you anything
because the rest are the only items you have to get in a particular quest, whereas you have to get 12 different potions for that quest.
.
because it's the only item that doesn't appear to be split in half.
because it's the only thing without a head/face.

