I like to pretend the Altmer in Oblivion is either a weaker, mortal version of Mannimarco. Since all instances of Daggerfall's endings occurred during the Warp in the West, perhaps both a divine and a mortal (if not multiple mortal) incarnations of him exist? I'm guessing he's weaker than his corporeal lich form in Daggerfall to begin with as he was still technically "unliving" in that state so this version of the King of Worms has actually lost power.
Another possibility is that he's an outright impostor.
I also speculate that, because of the Warp, that several mortal Mannimarcos exist. I brought this idea up in an earlier thread, and someone (I cannot remember who) said that the Warp only resulted in one God of Worms and one King of Worms. But as the Warp also resulted in a massive reworking of the High Rock feudal system (more than one region or kingdom gained power), I still find it plausible that more than one King of Worms exists.
But if I'm wrong, I can understand and agree with the idea that the God of Worms can manifest onto Nirn whenever he dang well feels like it.