Why are KDE and Gnome mixed together?

["Edward C. Jones" <edcjones@comcast.net>]

I installed stable (3.1) and got a mix of KDE and Gnome. I removed most 
of Gnome and upgraded (easily!) to unstable. I still get the mix. Why 
are KDE and Gnome mixed together? Where is this mix and its 
configuration documented? Why was this done?