The difference with the Koran is that it's much more recent than the others, and there doesn't seem to have been any significant change in the actual text since the time of Mohamed. (In one sense there's an analogy with the even more recent Book of Mormon, insofar as it does have a definite and recorded provenance. Though that's not an analogy to be stretched too far.)

Of course there has been a great deal of variance between the way interpretors at different times and places have explained that text and commented on it.