Why is distribution voltage 480V but equipment is usually only rated for 460V?
The feeder wires that deliver power have a small resistance/reactance.   When a current flows through them a percentage of the source voltage is lost to the wires.   So the voltage at load will be less.   120V at panel, but 115V at loads.  If the voltage is higher than 115V, it is just better efficiency.  

Electric codes set maximum loss to feeders.  It's how we size wires for installations.   

In your case, 480V at source, 460V required by load (nominal ratings) and a maximum of 20V lost to the wires. 

It's just Ohm's law applied to a series circuit with a small resistance (wires) and a large resistance (load).