Having never worked on A320, I can only comment that on aircraft I have worked with these graduations are used by maintenance to basically trim the engine to give an EPR or N1 figure commensurate with throttle position. With mechanical throttle systems on P&W engines for example a stop was inserted on the FCU,when the power lever in the cockpit was pushed fully forward to contact the stop, the lever was then adjusted to a given setting.The engine would then be run and adjusted to give the parameters required at that setting. I would imagine that with FBW systems it will be much the same but without the mechanical bit.