If you're talking about a wattage calculator (I don't like to visit sites I'm unfamiliar with) ignore them. They are not very accurate, and there is more than wattage. The primary rails are the 3.3V, 5V, and 12V rails. Increased load on the 3.3V and 5V means less potential current on the 12V. Also, as temperature of the PSU rises, potential decreases, so there are a lot of factors that play into effect here. You also start losing efficiency which also means more heat in ratio to the amount of DC and more heat = decreased lifespan of capacitors.