Prior the release of Windows 7, Microsoft was using "cute" names for OS releases. Windows 95, Windows Me, Windows XP, and Windows Vista. Windows XP was the direct follow-on to Windows 2000, which itself was a follow on to Windows NT 4. The first version of Windows NT was 3.1, and Microsoft chose that version number to prevent people thinking that Windows NT was a lesser version of the consumer oriented Windows 3.1. Windows XPs version number was 5.1, compared to Windows 2000 at 5.0. Windows Vista was version 6.0, with both Windows 7 and 8 only bumping it to 6.1 and 6.2 respectively. When it came time for what is now Windows 10, the choice of Windows 9 could have caused confusion with Windows 95, so they went up by one number. It has no relation to the naming of macOS.
For informational reasons, one of the reasons that Apple iterated 68K macOS to 9 so quickly, was to stop the legal mac clones. This happened when they brought Steve Jobs back, with the acquisition of NeXT, and the OS that ultimately became macOS X.
Resources:
en.wikipedia.org
en.wikipedia.org