Powering Your PC: Truths and Misconceptions
A Guide To Power Supplies

Output Voltage Regulation:

Power supply design is driven by two factors, size & cost. Using dedicated circuits and components for each voltage rail (FET, transformer, etc.) would allow for exact voltage regulation to be maintained under any load condition, however it would would be highly impractical  (both in terms of size & cost). This is why the ATX 12V v2.01 specifications⁵ allow for a ±5% tolerance on the primary voltage rails, to accommodate for the infinite and unique load differences each PC can create.

Also worth noting is the mention that "voltage regulation limits shall be maintained under continuous operation for any steady state temperature and operating conditions specified in Section 5." If you download the PDF you can view the mentioned operation conditions, the most important being the maximum operating temperature which is 50° C. Just keep this in the back of your head for now as we will talk about it more later.

The latest ATX 12V v2.01 Design Guide⁵ illustrates the following example cross regulation graph for a 400W power supply. Essentially, the graph is stating a PSU compliant with the 2.01 specs should be able to maintain the ±5% tolerance for any combination of loads on the various rails that fall within area created by the blue lines. Yes, some power supplies are rated for higher wattages & some for lower, however the general shape of the graph is the roughly same, the numbers scale up/down.

Power supplies today rely mostly on the 12V rail for the high current demands of components like the CPU, video card(s), and hard drives (to name a few), with 3.3V coming in a close second for components like memory and PCI cards. The 5V rail has slowly been de-emphasized in favor of the other two rails, however it is still used by a fair number of components (like USB and some onboard chips).

Older power supplies that were designed around the v1.3 specs have a different looking cross regulation graph because PCs from those days relied a lot more the 5V rail for primary power and 12V was not used nearly as much.