First of all, is your sub 2 ohm? If not its only going to use half that 400 rms anyway. I am sure your paper is correct putting it at 500 watts, or close to it. My general rule of thumb is that you are always better off over powering, rather than underpowering a subwoofer. As long as you dont play it stupid you will be fine.
If the output was tested at 14.4V, the numbers are different in the car. The 14.4V number is the new industry standard, but it's not an honest way to rate car amplifiers. Basically, if this number was 14.V, it would produce about 350 Watts at 12.5V (in-car voltage). It's not a big deal I guess because you would not hear the difference in output anyway.
Some companies still use 12.5V input voltage to rate power.