In Harry Potter and the Deathly Hallows Chapter 26: Gringotts, when:
Harry, Ron, Hermione, and Griphook are trying to get Helga Hufflepuff's cup from Bellatrix's vault.
We see that:
[Hermione] raised her wand, pointed it at Harry and whispered, 'Levicorpus.'
However in book 6, when Harry discovers Levicorpus in the margins of the Half-Blood Prince's copy of Advanced Potion Making, it is written as:
Where "nvbl" means "non-verbal." Harry proceeds to try out the spell on Ron nonverbally, and it works just fine. In every other instance of its use (e.g. James on Snape in book 5, Harry attempting to use it on Snape in book 6) it is also used non-verbally. It seems to be implied that the spell is meant by its inventor only to be used like this; otherwise, why would he have bothered to note "non-verbal" when pretty much any spell (including the Half-Blood Prince's own Sectumpsempra) can be used silently anyway?
So then why does Hermione (who happens to pick up non-verbal spells incredibly quickly in book 6, and is better at them than either of Harry or Ron) of all people use Levicorpus verbally by whispering it? Is there any particular reason for it? Is it even technically supposed to work verbally? If so, why did the Half-Blood Prince explicitly note it as non-verbal?