It's due to how Valve conducts these surveys. Rather than pulling from the entire user pool at once, they rotate between different chunks of the userbase every month.
You'll notice this if you look at the system language stats, The Chinese speaking population jumps and falls by a rather large percent every month.

This also could explain why Windows 10 had another jump in users this month, From what I hear internet cafes are still a popular way to game in China. and I doubt many of these places have updated all their machines to Windows 11 yet.
