Why does "uname -o" return GNU/Linux instead of Ubuntu?

Ubuntu is a GNU/Linux distribution.

man uname tells me the -o options report

   -o, --operating-system
          print the operating system

To me that is a valid answer, Ubuntu shows instead who packaged up the GNU/Linux operating system you are using.

I'd usually use lsb_release -a to view details about what GNU/Linux I'm using, for my system I get

guiverc@d960-ubu2:~$    lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu Groovy Gorilla (development branch)
Release:        20.10
Codename:       groovy