Could be flase the xfx temperature and the fans speed?
Nope, readings are true. I also noticed that difference. Tested on two rigs placed besides each other: 580 xfx 8gb gts and 580 nitro+ 8gb. Energy consumption is the same but temps and fan speeds can differ much. Sapphire: 70 degree C, fan 20%, XFX: 70 degree C, fan 90%. There is nothing you can do, you have to accept it.