This is because all power supplies have an efficiency of 75-80%.
The "real" power consumption of the system are 315 watts, but if you have in mind the efficiency of a power supply that total wattage go up to 394 watts (take it as 400 watts)
That's why they are suggesting a 450 watts power supply, to ensure you are not going to have any issue running that card even if you're overcloking.