So why are they called unbirths?
Unbirths have births, as Nomura said, much like how Nobodies have bodies and Heartless are comprised of hearts.
The birth, obviously, is the replication process.
Heartless are named so because they're instinctive, apathetic creatures of the dark, ie heartless.
Nobodies are named so because they are nonexistent beings, ie nobody.
Then lets look at Unbirth.
It has a negative prefix ("Un-") like the prefix/suffix for Nobodies and Heartless ("No-" and "-less" respectively).
Meaning that, while they have births, the word "unbirth" describes an Unbirths nature. Un-born. Opposite of birth.
The antonym of birth is death.
Therefore, Unbirth are born as "dead" beings.
Nomura has said that a heart is "proof of life." Naturally, without a heart, a being is essentially as good as dead.
Nobodies aren't really "dead" though since they aren't alive either. Something that does not exist cannot be alive and thus cannot have a death either.
Unbirth are born into this world as real beings. But they do not have a real heart. It is as though they're dead.
this theory would have been good if the unbirth emblem was actually accurate....
Looks accurate to me. Even if it isn't though, I really don't care. I've already held this idea since I found out Xion was a replica. And I've found how it can fit very nicely into the story. The symbol interpretation, which is entirely subjective, is just supplementary evidence that I found by coincidence (wasn't looking for it in the unbirth symbol, it's simply what I arrived to).