It's supposed to be a different planet. In The Magician's Nephew we see Aslan, who is presented as how Jesus appears in that world, singing Narnia to life and calling forth the sun and stars for that world. In other words, it's the creation of THAT universe, as opposed to ours. As a matter of fact, Narnia is a flat world -- in fantasy, you can do anything! -- and in Voyage of the Dawn Treader they reach the "edge" of the ocean where it drops off like water pouring over the edges of a giant table.The author C.S. Lewis had Christianity in mind as the basis behind Narnia, but he assumes that God can create any sort of universe -- and in fact has created an infinite number -- so some of the worlds in those other universes have wonderful creatures and things in them that are only "imaginary" in our world.