That's not how it works, your conclusions are totally wrong. SATA and Molex risers use both 5V and 12V rails, combo and 6-pin risers use only 12V rails. NVIDIA reports completely accurate power draw, which includes the slot draw. You are actually measuring the difference between AC (wall) and DC (cards) power draw, which is all about the PSU's (in)efficiency.
The proper way to accurately measure riser power draw is by using clamps or run your multimeter on power draw (A) mode and connect the leads in series.
You people need to relax, why is everyone so quick to jump to conclusions. So maybe i left out some details. That is correct both SATA and molex connectors utilize multiple voltages but where do you think the 3.3v and 5v are coming from.....lets think....maybe its the 12v rail right??? No its the 5v or 3.3v rails!! Dammit people use your head...
Not factoring in the load demand of the P/S originally will change things based on how many cards are plugged in, I'm going to measure current and voltage directly with my Arduino board to get actual riser power and run the cards with SATA, Molex, and PCI power cables at different GPU power levels.