The best answer I've seen so far is that "Claude 3.5 Sonnet" is a brand name rather than a specific version. Not saying I agree, just a way to visualize how the team is coming up with marketing.

It was certainly named by some nerd: "(pushes glasses up) well, we only updated the quantized diffraction sorter, the 3.5 version number refers to iterations on both that and the field matrix array interpreter, so technically because the interpreter hasn't changed so we shouldn't upgrade the version". This engineer has never seen a dollar from a customer in their life.