With the cables I think you can go 1 of 2 ways:
1: Build in a large enough safety margin in the cable (Eg a 450-600W card running with the two plugs as per that adaptor).
Or
2: Run the cable closer to spec but monitor and control the power balance with safeties.
They designed the spec to run under 2 with some overhead on the 30xx series, which is fine. Then they just got lazy and or cheap and or hung up on a small as PCB as possible for flow through/aesthetics (or a combo of all 3).
If they picked 1 of these then there would be only the classic numbers of burn outs there have always been with the 8 pins.