I read in another post a while ago that there was some issue with a Windows update that caused it not to register as Windows. It was then corrected in a subsequent update. So the Unknown bump is just Windows.
Edit for reference: https://gs.statcounter.com/os-market-share/desktop/worldwide/#monthly-202301-202307
If you’re on the website, go to your post, click the 3 vertical dots so that it expands, and hit the trash can icon.