Just arbitrary hand-tweaking. It was chosen because I though it looked good. When doing procedural generation like this, you basically just tweak constants 'til it looks good. There's no deeper reason behind it.
I'm hoping that this kind of interaction will be part of one of our first encounters with aliens, sometime in the far future. The aliens would ask why we had done something a certain way and the human answer would be: "Because it looks good".
