User blog comment:IamJakuhoRaikoben/Chapter 345: Someone's Voice, Review./@comment-4489126-20130726195736/@comment-7418318-20130726210918

In a fantasy world with magic there could be possibilies to create undead with varying degrees of awareness and intelligencce, hence the "usually".

We also don't know how (or it) ghosts are tied to their bodily remains in Fairy Tail.