For example, in some distributions like Debian, the same CD netinst allows you to choose one or all of the desktop environments (Gnome, Xfce, etc...) but in Ubuntu you actually have to choose between different distros or forks (Kubuntu, Xubuntu, Lubuntu...)
What's the difference between having a distro like xubuntu and installing ubuntu and then doing something like
sudo apt-get install xubuntu-desktop
Or
sudo apt-get install xfce4
What's the difference between ubuntu with xfce and Xubuntu? Why are they called "flavours" instead of just different desktop environments?