Other possibility for water using less watts with same clocks. From the Anandtech Fury X review:
"a side note, as we mentioned in our architectural breakdown, the amount of power this card draws will depend on its temperature. 408W at the wall at 65C is only 388W at the wall at 40C, as current leakage scales with GPU temperature. Ultimately the R9 Fury X will trend towards 65C, but it means that early readings can be a bit misleading."
Lower temps is lower power consumption. While that's only 20W, consider that's only 40C vs 65C. Vega blower could run at over 90C, as Hawaii was 95C IIRC.