I can believe that work done by professional employees outside of work hours related to their employment might sometimes be works for hire, since they're not hiring professional employees. But that just covers work related to the employment, not everything the employer might do.

If you're an accountant for Bloomberg NY who somehow avoided any contract provisions affecting this question (itself an unlikely scenario), they wouldn't own any spare-time Kubernetes hacking you do, since that's not part of an accountant's job.

In California, a contract might be able to make the employer own the Bloomberg accountant's spare time Kubernetes hacking only if the courts decide that it's related to other parts of Bloomberg's business, which is hard to predict; indeed, parts of Bloomberg do deploy and hack on Kubernetes and use it to run important stuff for their business, but of course their main business line is financial news and not container tech.

The difference with New York is that sufficiently inclusive contract wording would be able to assign that to the employer even if Kubernetes were not related in any way to Bloomberg's business, unless NY courts would it unreasonable to apply that wording.

(For example, I doubt NY courts would find it reasonable to assign any employers copyright over an employee's written grocery shopping list, aside from household nanny/errand-runner situations.)

The work-for-hire rules are actually federal: anything covered by them is employer-owned by default in all of the US including California, not dependent on a contractual provision which California could choose not to enforce, with the employer being treated as the actual author for copyright purposes rather than just an assignee.

So the only relevant difference between the states would apply in cases where federal law doesn't automatically do that, where people incorrectly assume it doesn't do that, or where a contract or other agreement by the employer explicitly overrides the work-for-hire rule (if this is legal) or reassigns or disclaims the employer's interest in certain created works.

I'd be interested in a citation to either a court ruling, a statute, a regulation, or similar to clarify this as applied to software workers.