Astra > Sol > Terra > Luna seems clear enough to me
Leopard (10.5) and Yosemite (10.10) both came with clear version numbers
Model X and Model Y are siblings which do not have a clear hierarchical relationship
Gemini 4 Carbon and Gemini 4 Argon are rather difficult for me to tell at a glance
Confusingly, Carbon is another model in the same size class as Argon. Every other lab would have just called it Gemini 4.1 Argon.
AFAIK they are all (Barium, Argon, Carbon) different checkpoints of the same (ongoing) training run
Ideally, Google would have run with:
Argon > Neon > Helium
That would have been the noble thing to do.
Or the other way around! Radon would have been an awesome name.