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
Yep. That was my first thought - how everyone says to use Matrix rather than Lemmy DMs for anything sensitive. This will be fantastic.