Computer still using electricity even after being turned off (aka vampire power aka phantom power)

I checked to see how much energy was being used by a computer in our home. Not unusually, it was taking around 90W when turned on. What was a bit of a surprise though was that it also drew 3W when turned off. Turned off, mind, not put to sleep/hibernate. If this computer is left plugged in but turned off 24 hours a day for a week it would consume around 504Wh of electricity (the same as leaving a 100W light bulb on for 5 hours). Update: I checked the computer subsequently and it consumes 11W when put to sleep.