Not strange logic really (for once!) It makes sense if you think that some controllers have separate power supplies for analogue and digital outputs. For example, this allows you to use the DC output from the VFD to supply the analogue output circuitry in the controller. A lot of cheaper BOBs only have a 5V supply for the digital side of things and need an external supply of 12V or so for the analogue output. And after all, it just needs an extra wire to make it all work with a common ground! The info is on the circuit diagram, although it doesn't leap out at you and I don't think that there is anything specific about it in the HY manual text.