I just realized that they seemed to have either done away or forgotten that the whole game had originally been framed as a story Kairi's Grandma was telling Kairi.
The story from her Grandma still applies to UX, it just wasn't show, really. She talks about how the children rebuilt the world after it was engulfed by darkness and resulted in the smaller, separte worlds we have now: After the original world is destroyed (final scene of Ephemer and Skuld looking at Daybreak town), said event probably occured where the children that lived elsewhere and had nothing to do with the whole X mess rebuilt the world.
Some time later, Ephemer is floating around in his Lifeboat in the ocean of the remains of Daybreak, which now most likely functions as a individual empty world. He rebuilts said world into Scala (I assume in a similar fashion to how Land of Departure turns into Castle Oblivion and the other way around, tinkering with the world's heart to reshapee it), yada yada.
Daybreak was purely the center of said original unified world, not the entire thing. They just did not bother to show any other part of the world besides the desert that becomes the Keyblade Graveyard, and those mountains where the Dandelions met with Ava.
I doubt we would ever get back into this unless they want to navigate around the mental image of said unified world. lol
Ansem Seeker of Darkness' design is arguably an anomaly because Xemnas was clearly modelled after Terra (or the other way round if you really want to get technical about origins) because Xemnas is the Nobody of Terra-Xehanort. Terra's heart clearly had no influence with Ansem Seeker of Darkness so I think his design is what an Adult Master Xehanort would have looked like.
Personally I think they both look the same, it's just the outfit that changes (with SoD wearing the exact same fit as Hag Xehanort, just with his massive titties pushing the buttons open). But with Nomura's artstyle and everyone looking the same, does it even matter?