Perhaps the game has been programmed with a set of tables that give the atmospheric / hydrological / lithological contents based first upon planetary type. It would kinda be like the StarGenerator program I recommended to joke_dst a few months back...you can have all other indicators pointing to a habitable planet, but if the desired type is wrong, you won't get a habitable planet. I'm just amazed something as early as SF2 could be that advanced dynamically.

It might be worth experimenting with the planets a bit. Here's something: try changing a world to the same settings as another world (leave its position data alone, but change everthing else to match the other world), and see if you end up with two identical planets. Better still, see if the planets are exactly identical, down to their geographical features. It would give us an idea of just how much information is contained in a single planetary reference, and it might fill in some of our knowledge regarding "missing bits" and so forth.