Earlier quoted context omitted.
It's a combination of (1) and something you don't list: I think the frontier labs all have multiple generations of undisclosed models in continuous training. There is no "end point" when it's magically "ready". It's just getting better and better all the time. What they release with a name and a version number is just a marketing / branding exercise. So what you experience as a "near simultaneous" release is just the…
That’s not how training pipelines work, and would be extremely wasteful for the biggest cost center as well.
So Model N/N+1 might literally have had the exact the same pretraining run and only differ on how much/what kind of postraining they got