"That's all very well in practice, but will it ever work in theory?" -- G. Hill
"A tidy laboratory means a lazy chemist." -- Jöns Jacob Berzelius
As Laserjock pointed out, Kubuntu/Xubuntu/Edubuntu/Ichthux (as of version 6.10) use exactly the same repositories as Ubuntu. They are the same OS, with the same packages, the same developers, the same standards, the same Code of Conduct, the same release schedule. It is a common misconception that Kubuntu/Xubuntu/Edubuntu/Ichthux are derivated from Ubuntu. They are fully part of Ubuntu.
However, Kubuntu, Xubuntu and Edubuntu are officially supported by Canonical, and the trademarks belong to Canonical just as Ubuntu, while Ichthux is a community-only project, that has nothing to do with Canonical.
I kind of agree that the names could have been chosen more wisely. It seems this began with Kubuntu, and just went on with the other projects. There are pros and cons with these names. I think it can be good to have different names, since most end users really see different desktops. Ubuntu and Kubuntu look & feel different, so it is natural for people to use different names of that. Advanced users are not shocked to find out they are the same distro, and get along with it. In this matter, Kubuntu/Xubuntu/Edubuntu/Ichthux are indeed various "editions" of Ubuntu since they use exactly the same packages, tools and repositories but only ship different packages by default. The Kubuntu/Edubuntu/Xubuntu/Ichthux developers are mostly Ubuntu developers, and work directly in Ubuntu. Ubuntu Christian Edition cannot be considered an Ubuntu "edition" in the same sense though, because it is based on Ubuntu but not included in it.
Ichthux Project Leader
http://www.ichthux.com - Linux for Christians