Rated power is for cheap or old amps that don't have regulated power supplies. If the amp had a regulated power supply, the output power would be the same no matter what the input voltage is.
Most batteries put out about 12.7 volts, and the alt can peak at 14.4 easily. Ideally, your actual voltage in the car is on average about 13.8-but lower if you're running too many amps or just drawing more power than the alt can handle.