Why would they differentiate between Debian and Ubuntu users like this since Ubuntu runs on Debian?

Of all Linux users, 38.2% use Ubuntu as of May 2019.

21.5% of users rely on Debian.

What I also do not understand is why Chrome OS. and Android, which run on the Linux kernel,why are they not put under the Linux umbrella ? My best guess is the writers do not know the above and it never occurred to them to find out. Or there is an anti Linux bias (most likely). These are your main OS choices:

Windows (Originally DOS)

Linux kernel (everything runs on Linux,most phones,smart tv's,microwaves,NASA'S stuff,the Internet,this website,etc,etc,etc.)

BSD (Apple,Free BSD).