400V +/- 10% means it's designed for a nominal voltage of 400V but the actual voltage can be up to 10% above or below the nominal value without damaging the equipment. This tolernace is intended to compensate voltage drop due to feeder length, voltage fluctuations under heavy load,... and the US 480V sure have a tolernace range too! (IIRC the US 120V are +/- 5% for example). So if you take a 5% tolerance the 480 might actually go up to 504V! And that will for sure kill the 400V machine.
