Yeah, I think it's primarily just there as a poor attempt to deal with inconsistencies (Tiamat as Takhisis being a Greater Power).
The thing is, in my world deities are NPCs, like they've been officially in pretty much every edition of D&D (2e was weird, but it talked out of both sides of its mouth a lot with contradictory info). They have stats. They can talk with you. So the question is, Tiamat runs into Moradin in the Nine Hells, and what rank of deity is she? All this stuff about being relative to a certain world makes no sense when you are actually dealing with an NPC at home that has an actual divine rank. If they had decided that divine rank had nothing to do with the being themself, but only with how much power they could exert on a world, then it would make perfect sense, but that is basically the opposite of what they did. There is no description of how a Lesser Deity has any less influence on a world than a Greater Deity. They all grant the same spells. There is no rule of the game, or even described fictional reality that makes their influence on a world vary in the least based on that rank. What varies is whether they have* a physical form (which usually isn't on the world they are worshipped on!) they are using to sit on their throne in Arborea or not, and whether they are comprehensible. None of that has a thing to do with worshippers on different worlds.
In the example of Takhisis and Tiamat, the way I deal with it is that Takhisis, the important Greater Power of Krynn has a permanent avatar created way back when, that has since become a semi-independent Lesser Power known as Tiamat. With regards to deities that don't have enough going on to justify that sort of interpretation, I'll probably just have to say that they actually are the higher rank, but don't get as much respect on some worlds or something.
*I would say, "are necessarily bound to", but again, that's me fixing a sloppy sidebar not what it says.