I believe the default power consumption of rx 580 is slightly higher than rx 570 by about 20w(I might be wrong) but I am sure they where higher, I believe that explains the higher power draw at the wall.
Regards,
BLife
this
http://www.anandtech.com/bench/product/1869?vs=1868Load Power Consumption - Crysis 3
Total System Power Consumption in Watts (Lower is Better)
rx 570 297 watts
rx 580 324 watts
Load Power Consumption - FurMark
Total System Power Consumption in Watts (Lower is Better)
rx 570 237 watts
rx 580 250 watts