Centaurus my guess for that would be , since N1 is an actual measure of Compressor speed , that turbulence penetration power setting would ensure a better avoidance measure of compressor stall ( that is a partial breakdown of airflow within the engine )
Disturbed air (from turbulence ) and rapid advancement / retardation of power levers , results in the intake air not passing over the compressor blades at a designed range of angles which ensure optimum and smooth operation of the engine . The manufacturer would recomment a engine speed that best assures a minimization of this situation should the aeroplane enter turbulance conditions.
Again i would not like to be categorical on why N1 is used instead of EPR , but since EPR is more of a thrust setting , it does not serve as well as a compressor speed setting for this purpose .
More jet type experienced posters may correct me.