Shoing 275 Watt and showing 275 Watt isn't the same, with 2 different PSUs.

If its efficiency is 50% that means the GPU is pulling 137,5 Watt, but your PSU consumes 275 Watt, the difference is mainly going into heat.

If your PSU on the other hand has an efficiency of 80% hen your GPU is pulling 220 Watt.

So the Kill-a-Watt only shows you what bill you are going to expect, not how much the components are eating up.