The PMD itself is not 100% accurate. It's using software to estimate power draw rather than something like a c clamp. However it's likely more accurate than the oled itself if you could connect all the 8 pins AND the pcie express slot.
Cards don't evenly draw power across all the connectors. Unfortunately the best you can do is make an educated guess. You don't actually need to plug in all 6 8 pin cables. As long as both 16 pins are plugged in you can run the card just fine as long as 3-4 8 pin cables are plugged in. I've benched my card with just 4 8 pins connected. If running ln2 I would suggest using all 6 but for the kinds of power draw were using on water we won't need more than 3. The card was designed to run on 3 8 pins just like the 3090. Nvidia was the one thar forced EVGA to adopt the dual 16 pin setup as that's what they're using on 3090 Tis and likely all future 40 series cards.