GPU-Z only shows the actual power draw of the GPU core. There is some lost in VRMs, memory system, PCIE interface, fans, and system PSU.
If you have cards with proper VRM monitoring available(like polaris nitro+ or refs) you can actually see the memory part consumption in HWiNFO64 and adding up core and vrm parts and ~40W for a system makes it pretty close to the actual consumption from the wall. (not sure what to make of it, but there's no missing ~10W per card for fans or something)