The single chip ones were not originally supposed to be PDP-11 compatible - but there was a push to use PDP-11 ISA and Unibus/Qbus in that specific segment, so

The single chip ones were not originally supposed to be PDP-11 compatible - but there was a push to use PDP-11 ISA and Unibus/Qbus in that specific segment, so they reworked the microcode to implement PDP-11 instructions with minimal changes to the chip itself.