Is the "underworld" map in Terranigma accurate with the storyline?

Something bothers me about the depiction of the "underworld" in Terranigma.

So in the dialog of the game, the world is described as a hollow sphere wherein the underside is the "under world" and the outer side is the "surface world". This is backed up by the part of the storyline where you go from the under world to the surface world by jumping through a big hole in the ground (or "portal"). The above description would look something like this:

However when you leave Crysta early in the game, the world map is depicted thus:

It clearly looks like you are walking on the inside face of an outer sphere, with there being two concentric spheres, and the internal one shows the oceans of the 'surface world'. In such a setup, things would look more like this:

So which structure is correct? Did the graphics people just screw up the look of the under world map at the beginning of the game or am I misunderstanding about the structure of the world? :-)