It's a very good question to ask, too. The four northwestern provinces are very hilly by the terrain visible in this map, rather than mountainous as I described them (an error in generating my terrain which should be rectified in the next version), but those most western areas do tend to have both richer soil and higher rainfall than the central plains. Much like most of the central plains in the USA, the plains of Atrusia in the west-central area get a large portion of their water as groundwater or from deep wells, rather than getting heavy rains. In addition, the drier plains should be more greenish than they are right now, which is a coloring issue.