You're correct in what you say. But the only thing that would make sense is if the board detects that the memoy isn't compatible for the chosen voltage and tries the closest spd profile, the sensors are showing incorrectly or the bios firmware is messed up. You could try to measure power consumption while changing between 1.5 and 1.35 to see if it actually works.
Spoke to support via Live chat a little, and from what I take away from that is that is BIOS option does nothing and thus is bit deceiving and just there to cause confusion it seems. Was told to try call local support to try escalate this issue, but not sure if it worth it.