This might be a little far fetched considering that I'm half asleep while typing this, but wouldn't it be more logical if an Unbirth was the physical manifestation of the lack of a body, but the remains of a heart and soul?
Kingdom Hearts antagonist labels are extremely contradictory, in case you haven't noticed.
A Nobody HAS a body.
Heartless HAVE hearts.
An Unbirth could play a reverse role and contradict it's existence ; if the heart and soul are still in tact with each other, but have no vessel to fulfill their duties in , what if they... created their own?
The heart and soul would make it so that something is already there, but not, at the same time. The person lost both, but they're drifting.
So something is put in placement of the body.
Don't think of it so much as an Unbirth, but more as of something that forced it's own rebirth.
Tetsuya DID say that Unbirths should be viewed as something to the effect of the exact opposite of human life.
Humans, even in Kingdom Hearts, cannot force their own existence.
So this would fit the description of our opposite, eh?
Sorry if this didn't make sense, lawl.