It is commonly believed that there is a sort of stratification among the gods. Some are considered full-fledged gods, while others are considered sub-deities. There seems to be a unique reasons for each of these being lesser than the others. However, a common theme seems to be the nature of these things, it is unclear if the deity is 'master' of such a plane (often called a demi-plane or sub-plane) because it isn't powerful enough to control another plane, or if the plane is a lesser plane only because of the nature of the deity that rules there.
H.A.M.