Right, but why? Was it just decided that the arctic should be in the middle? Or, is there some reasoning behind it?
Whatever the FE-ers tell you, it's bullshit. The FE maps are simple projections of the RE Globe since they have no means to chart the whole world on their own. They chose an azimuthal projection with the North Pole as a central point. Now, the distance on the FE map between any point and the North Pole is the same as the true distance between them on the RE. This is why the FE has a diameter equal to the length of the RE great circle (which is approximately 40,000 km or 24,900 mi) and the Equator is at half the distance between the North Pole and the Ice Wall.
There is a mathematical theorem proved by Carl Friedrich Gauss, called
Theorema Egregium (‘Remarkable Theorem’) which implies that an undevelopable surface (like a sphere) cannot be laid over a developable surface (like a cylinder, a cone or a plane). Also, there is a topological inconsistency, since a sphere is a topological sphere and the plane isn’t. In lamer’s terms, this means that one has to ‘cut the sphere’ in order to lay it flat.
So, there has to be a compromise. The best place to cut the sphere and to allow for the greatest metrical inconsistencies is where there is the least population. Two places on Earth fulfill this condition: the Pacific and Antarctica. But, once you choose this place, your map would be most accurate at the diametrically opposite area. The opposite of the Pacific is somewhere in Africa or the Middle East, and the opposite of Antarctica, is, of course, the Arctic region, which, conveniently, is where most of the Western Civilization is. Hence, the North Pole was chosen as the center of the disk.
Another convenience is that Antarctica is covered with ice and this can be used to hold the oceans’ water from spilling on the FE.