The numbers are branding. The appear to be an indicator of a given year long training run. New “versions” are tweaks of the same base.

Sure and that is why you can call it 2.5.<whatever>

They just don't want to be pinned down because the shifting sands are useful for the time when the LLM starts to get injected with ads or paid influence.

I wish they would actually explain it like that somewhere. Or publish the internal version numbers they must certainly be using to ensure a proper development process.