I mean, what really is the difference between a PDP-6 and a PDP-10 or DECSYSTEM-20? From the programmers' point of view, not much.
The same goes for PDP-4, -7, -9 and -15.
The same goes for the PDP-5 and -8, and Harris 6120.
For whatever reason, the PDP-11 stayed as -11, despite the huge variation in the product family.