I'd say that the AI that came with Kal-El's rocket presents to Superman an image of Krypton that he can understand. So it reads his thoughts and then feeds back a representation of the world that Clark will accept as true. But as the Man of Tomorrow's outlook develops, how he sees Krypton keeps changing. And the Fortress of Solitude just keeps manifesting those different ideas of what the Action Ace thinks Krypton ought to be. So the Fortress never remains static.