The rating of any power supply is governed by design and components......
For example, a linear power supply running from a 240v ac supply,could be pulling as much as 1 amp(AC current ) giving an output of 12 volts @ 10 amps would be designed to do that.......although if it werent loaded to its full rated output, the consumption of the supply would be a lot less, than if it were fully loaded.
Switch mode power supplies draw the same parallel.......less load, less current draw from the primary supply......be it a computer power supply or whatever..
The main advantage of a switch mode over its linear counterpart is the fact that its more efficient....
HTH Chip