As far as I know, all PC PSU's use switch mode power supplies. This means that the mains input is rectified to produce a DC voltage which is then chopped up at high speed so that the transformer can be much smaller than if it was driven directly from 50 of 60 Hz. The switching is also controlled to give a constant output voltage regardless of variations in the input voltage. This generally only accurately controls one output voltage and the other outputs are therefore subject to greater variations.
Antec make what they call the TruePower series where they more accurately control all the outputs with individual regulators for greater accuracy. click here