CCleaner is reporting Mac OS X 10.16 whn current version is 11.5.
Mac OS X 10.16
MacBookAir 9.1, Quad-Core Intel Core i7 @ 1.20GHz, 8.0GB RAM
System Software Overview
System Version: macOS 11.5 (20G71)
Kernel Version: Darwin 20.6.0
Boot Volume: Macintosh HD
Boot Mode: Normal
MacBookAir 9.1, Quad-Core Intel Core i7 @ 1.20GHz, 8.0GB RAM
It isn't a bug, it is intentional by Apple.
Big Sur is offically both v10.16 and v11.#, which one you see depends on where and how you look.
It's for legacy/compatibility reasons, see this for a fuller explanation: https://community.ccleaner.com/topic/60444-need-help-understanding-top-right-corner/?tab=comments#comment-328971
Thanks for the complete and interesting answer. I figured it was not just simply overlooked.
P.S. I too, have a taste for beer, Guiness is a favorite, and Kentucky whiskey or a very good Scotch, within my meager limits, that is,