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.

"Argon" had me thinking they would go with a noble gas theme, with Helium as the light one and Xeon as the heavyweight.