Because the total Wattage of a PSU is not broken down for the consumer - Amps are what you need to look at - end of story. Some of us know how to do calculations for current voltage and power but all GPUs need a powerful 12V supply and the important spec is how much current on the 12v rail - not the total power of a PSU!!!
-Bruce
Edit: BTW good example gopher 36A VS 54 Amps on the 12V Rail - I would pick the Seasonic 650W!