My guess, by atomic weight. Hydrogen = "sonnet". Carbon = "opus". Argon = "Fable" only argon 4 just happens to be kinda shit? But also it's google so who the hell knows. They could be just going alphabetical argon, then carbon, then hydrogen as release order follows the alphabet. That'd be kinda dumb but so is their access/subscription strategy so it'd be on brand.
I thought they were going by alphabetical order like intel does for GPUs
A = argon for initial release B was skipped because you can't name a product boron C = carbon D = ??????
They might be going for atomic weight, but the average person couldn't say whether argon or carbon was heavier.
Reasoning-wise Carbon > Argon (so actually is Carbon, Argon = "Fable", "Opus").