My pet theory was that they wanted to stay on the next version indefinitely but Mac OS seemed to be indefinitely on version 10, and they couldn't be one behind. Funnily enough, soon after Mac OS went to version 11, Windows 11 was announced.

(realistically though, the '9x' problem does make a lot of sense)

An alternate reason that doesn’t require such stretching of credulity around one broken piece of software is that MS already had two products starting with “Windows 9” and that people were likely to confuse them, whether end users or someone updating a catalog.

TL;DR: Some code looking for Windows 95 or 98 only looked for "Windows 9" to match both, and would have matched Windows 9.

Now Apple has the year in the OS version we'll have people wondering in a few years what happened to iOS 19-25.