I think we're using the same words to describe different things. I'm using the word "setting" merely describes a small part of a unique game
universe.
In Traveller's case the Marches, Spica, Gateway, and the Rim are different settings - but it's still the same
universe whether you want to define it that way or not. it's all part of Charted Space.
The thing with D&D is that Faerun (FR), Greyhawk, Krynn (Dragonlance), and Athas (Dark Sun), Mystara (Basic D&D) and Eberron are all different worlds and different universes. But the rules concepts are all compatible, because they're all based on the D&D ruleset (some emphasise certain aspects more (eg Dark Sun with its psionics) and lack others (eg only Eberron has the clockwork/mecha elements). All of the above are D&D, but they're still not all the same universe.
In Traveller, the closest equivalent will be with QLI's approach: T20 is the core rules, but there are different settings: Gateway/1248, 2320AD, Honor Harrington, and Aldenata. You could say that all of those are Traveller (because they use the same rules), but they aren't all the same universe.
The Marches, the Rim, Gateway, Spica etc are all different "settings" in the same universe of Charted Space. They share a common history, the same races can show up, the same ships ply the spaceways etc.
Sure the same rules apply, sure the same physics apply, that doesn't make them the same place.
Of course it makes them the same place! It's all set in the same universe - "Charted Space". The one with the Imperium and the Rim War, Beowulf traders, and the Aslan and Vargr and K'Kree etc. Whether any of those show up in the games is utterly irrelevant to that definition.
Star Trek: TOS, Next Generation, DS9 and Voyager are all set in the same universe. Sure, they're set in different locations and very different in feel and there's very little overlap between them and you can call them different settings, but they share a common background and the same technologies and races - because they're in the same universe.
Calling Greyhawk and Forgotten Realms different settings is no different.
It's very different - especially if you use an example of Forgotten Realms vs Dark Sun, which definitely ARE different universes.
(WHFRP) Or Middle Earth. These are truly different.
Of course they're different - they don't use remotely the same assumptions, mechanics, or setting. They are not the same universes.
But saying these D&D settings are different but the Traveller settings aren't different makes no sense at all to me.
Everything currently published for Traveller is still aimed at simulating the same universe - that of Charted Space. The "feel" of it is irrelevant - the fact is, you can point to a common map of Charted space and find the Marches, the Rim, Gateway, Spica, and all the other sectors. That's what makes it the same universe, and that's what is important in defining all the settings like Gateway and the Rim.
It does not take different planets to be different campaign settings. It doesn't even take different Universes to be different campaign settings.
Admittedly, Spelljammer tried to make all the D&D settings part of the same universe. In a sense, they are because you could (with a bit of rules adjustment) take a character from one setting and transport him to another via Spelljammer. But for all other practical purposes they're separate universes in the same way that Heavy Gear is a separate universe to Cyberpunk 2020, Blue Planet, Transhuman Space, and Charted Space.
Lets use a different analogy though. Just because they are on the same planet, in your opinion would New York City, Austin, Texas, London, England, Hati, Baghdad and Beiging be the same or different campaign settings? Or sticking to Strictly one language well mostly one language, London, England, Belfast, Ireland, NYC, US, Los Angles, US Honolulu, US, Sydney, Austraila be the same or different if you were to run a game in those locations?
They're different settings, but they're still the same universe. Someone from England, Papua New Guinea, Texas, or Bolivia could show up and tranfer beween any of those settings, because they still exist in the background there.
The FRFZ, Bug City and Seattle were all different Campaign settings in Shadowrun, but it was fairly easy to travel between them. (And they were all in North America.)
You just shot your own argument down there, didn't you?! That's exactly the point I'm making - different settings can be in the same universe. If you accept that the FRFZ, Bug City, and Seattle are in the same universe (the Shadowrun universe), then you can accept that Gateway, Tte Marches, and the Rim are all in the same universe (Charted Space).