Integral map designer documentation?

I just bought Sins a week or so ago, and I'm favorably impressed.  Doodling about with the map designer, though, is a little confusing.  What is the relationship between a "planet group" and the planets that show up in the map?  Even when I set the min and max values to 1, I get more than one planet, of types other than the type selected (i.e., if I specify a terran planet, I might get a terran and a couple of volcanic worlds, a terran, a volcanic and an ice world, etc.).