Chip's power consumption is <6.6W/Ghs
ATX power loses(82%), DC/DC power loses(87%), Fan(5~20w), etc
So
650W * 0.82 * 0.87 - 20W =
443W, About right.
This is a business opportunity to make a high efficiency wall to dc converter.
Making a very focused product, well specified load and voltage, can allow the efficiency to be much higher.