Ok, this is a rather odd question about the spells that both the arcane and divine users cast.

As we know, both the arcane and divine spell categories have their own unique spells. However, some spells are shared by both categories(I.E.: Both categories can use command, but only clarics can use greater command; both categories can cast dispel magic and greater dispel magic, yet only arcanes can cast Mordenkainen's Disjunction; both categories can cast energy drain but only arcanes can cast enervation, exc.

My question is, why in the heck are some spells shared by both classes, but yet lesser(And greater) version of these spells are only exclusive to that class?

Oh, and P.S.:Who exactly is Mordenkainen?