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.