IIRC Ken originally had statically named the serial devices for Unix although I’ve forgotten the assignment but the DL11 was tty8 and tty9 and DC11 started at 0.   DL11 were terrible I/O devices which is why the DH was created.  The idea from DEC was that you were supposed to use DH/DM pairs to support your terminals and modems on pdp11s.  

I suspect Ken/ BTL was following the DEC hardware rules and just modeled the Unix naming to match. 

In practice most sites had a single KL as the console and the used DZ and DH’s and had a script/makefile in /dev to create banks of device 8 or 16  at a time.  

Btw. I’ve forgotten the details the DC as we did not use them at CMU nor Tektronix or UCB by the time I got there  [We used either real DHs sometimes DZ but not if I could stop it or at CMU a locally created device called a ASLI. Later once they came on the scene most Unix sites I knew used Able DHDMs because they were cheaper than DZs, took less bus  space (single card for 16 ports) and of course were DMA so they worked better.   

FYI not to complain my small disappointment with simh has been the DH emulator.  The sim folks are assuming dz so never really worked to fix it.  It’s on my Long want to do list to figure out why.