If I had to guess, System 10 was a failed project fork by the game devs.
Happens quite a bit in tech development. You get so far down a certain path, realize that what you were trying to achieve is not going to work (dead end), write off the project and start over with a new Version #. Mostly, to avoid any confusion between plans or stored files that were already labeled "Sys10" getting mixed in with the new Sys11 development.