ATX only allows 5% on the 3.3V, 5V, and 12V lines, it allows 10% on the -12V rail which is rarely used, so the limits on the voltage ranges are 3.135-3.465V, 4.75-5.25V, 11.4-12.6V. Ideally you want it to be close to its nominal values so the voltage regulation circuitry for each component doesnt have to work as hard, but they can usually handle just fine right up to the edge of ATX spec.
The reason the voltage changes is that when you draw more power you have more current going through the wires so you have a larger voltage drop, the power supply will usually slightly increase the voltage on its end to compensate for this but its rarely perfect. From the numbers you posted your 3.3V rail is about 3% regulation which is decent and your 12V rail is a bit over 3%, not too bad, certainly nothing to worry about it wont hurt anything, assuming those values are true. If you are measuring them with software they likely arent accurate. BIOS measurements are usually pretty good but a digital multimeter is the only way to be really accurate.