I think those were release years. More like how we went from Windows 8 to 10.
(I understand that the real reason here is either a) OSX went to version 10, so don't want to be behind, or b) many systems would pattern match on windows version 9*" so Windows 9 would be treated as 95/98.)
Examples of B were found in open source, including some versions of the OpenJDK.
Mac OS was said to skip 9 on the way to 10 due to a combo of some Asian numerology (just as Western numerology often encourages skipping "unlucky" 13) and also trying to make a clean break more obvious between System 8 and OSX. (The X being the 10, as Apple's favorite way to write 10 for various reasons. Which is why "OSX" was always a misnomer, accidentally burning the version number into the operating system name. Part of why Apple has been trying to get everyone on the "macOS" brand lately to pull the version number back out.)
https://en.wikipedia.org/wiki/Mac_OS_9
Uh, Mac OS 9 existed. It shipped with early iMacs before OS X was stable and for people who needed to run old apps natively.