Cinnamon 6 and the beta of KDE 6 both arrive at once
The overlap is just a coincidence, and the two projects have differing approaches to versioning. Cinnamon is much younger: the project began in 2011, based on GNOME 3.2.1. KDE, on the other hand, is 14 years older: the first beta was in 1997, making it one of the oldest open source desktops. It's not based on any other desktop's code, but it uses the Qt toolkit, whose first release, 0.90, was just two years earlier.