Yea, they spawn from the same stuff but then go totally different directions. "The only real difference" is a dang big difference...
And as I said in another thread, its that they "know everything" that crucially separates the immortals from the mortals. The body and soul are irrelevant, the only thing that matters for immortality is that the consciousness and memory remain (as it is the memory that makes the person everything that they are)... To quote myself "If I'm striving for immortality, it really makes no difference to me whether my body or soul stay intact, but if I lose the thing that makes me me (memories, consciousness, etc) then I'm just as far up a creek as everybody else..."
Parts of themselves, that's where we get earthbones...
Even still, the very concept of "mortality" is moot--- By having existed, they have always existed, and will always have existed. If you remove the convenience of time from the equation, mortals are just as permanent and troublesome to remove mythically as are gods.
I exist, therefor I am.
Being run-over by the metaphysical machinations of the mundus is what really causes death. If we take some of the creation myths into account, we can deduce that this same doom machinery can digest gods just as easily. However, The knowledge is not TRULY lost-- it is retained by the universe as history and myth, even if everyone forgets about it happening, or existing. In the case of mortals, Nirn itself has the "Perfect memory" required. This is hinted at several times in in-game literature, be it from the spore dreams of moth priests, to practitioners of mysticism accessing it to identify dreugh puke balls.
Immortality is just a stone's throw away on Nirn. The problem, is that everyone wants to live as a
mortal 'forever'. The mortal existence itself needs to be shed to live in the immortal now. 'The temporal myth is man' as Vivec would say.
To live as an immortal, you would have to 'live' inside the dream-sleeve, and simply cause a mortal avatar to be reborn for you. You would experience through this mortal avatar-- which would die and rot-- as all mortal things do-- but "you" would continue to persist inside the perfect memory of the mortal world. Who is to say that the mortals on the mundus are not actually immortals, who purposefully withold perfect knowlege from their avatars, simply so they can experience novel things?
I remember watching a brittish space-opera sitcom called Red Dwarf-- where the ship's AI had been drifting through space for over a million years, and had read EVERY book in the ship's database, and had one of the crew members purge a portion of it's memory banks so that it could 'forget' having read them, and enjoy reading the books once again. If you think about the "Destruction" of a mortal's memory in ways like this, you start to come away with wholly new perspectives on mortality. A divine being could enjoy an otherwise tired old flavor, by revoking the past life memories of its mortal personifications, and in so doing, experience it as if it were for the first time, OVER and OVER and OVER again. This, I think, is the reason for mortals being denied memories of their past lives--
It is also kinda why many divine beings who center around "cycle of life and death" (Arkay, Meridia and friends) STRONGLY dislike necromancy, or mortals attempting to circumvent reintegration with the dream sleeve, or at least I think so. It robs the TRUE divine of being able to become 'new' again (because the 'old' never brings its memories home again.) It also seems to comply with the "schizophrenic ball of insanity" that Mundus as been described as prior.