Not sure why this is still a question (goes beyoond just HF).  Remember even excluding water cooling pumps, fans, and controller boards there are a lot of efficiency loss between between the wall and the chip.  The chips doesn't run on 12V they run at ~1VDC (exact voltage will depend on ASIC design) and no ATX PSU supplies 1VDC so ANY ASIC will need to convert the 12VDC to ~1VDC used by the chip and that usually means at least a 10% loss right there.  Of course most people don't have 12VDC outlets so you need to convert the wattage at the wall to the 12VDC uses by the ASIC board and that is another 10% loss (depends on PSU rating and load).

So a hypothetical chip (any chip) which uses 250W @ ~1VDC will require at least ~277W @ 12VDC not 250W. ~27W is lost as heat not at the chip die but at the DC to DC PSU.  Now for the ATX PSU to supply 277W @ 12VDC will require 308W @ 120VAC and another 31W will be lost at the ATX PSU.  Add in 20W for pump and radiator fans plus a 10 to 20W safety margin and ~350W system wattage with 250W chip wattage makes perfect sense.  Once again this isn't unique to just HF, it applies to all electronic devices.

Case in point.
Avalon chip wattage: 1.86W @ 1.15 (282 MH/s nominal)
Avalon total chip wattage 1.86W x 212 chips = 394W
Avalon wattage at the wall: 620W
Total system wattage is >57% more than chip wattage.

https://en.bitcoin.it/wiki/Avalon#Power
http://garzikrants.blogspot.com/2013/02/avalon-miner-power-usage.html