Yeah, it's random, and it's also based on the distance between settlements. So if your world builds up very evenly around themselves, the potential distance between settlements goes up much more slowly. For worlds that skew in a longer, thinner shape rather than a fatter, rounder shape, they will have more settlements sooner. On the other hand, the fatter, rounder worlds would have more settlements show up overall as time passes -- suddenly a rush of new settlements all over the various border areas.