Having seen the sometimes rather different N1 indications for equal EPRs on the RR Tay 620 and 650, I have to draw the conclusion that EPR tells the truth about engine power output while N1 allows a rough estimate only. During takeoff/climb, obviously the engines were adjusted to produce equal EPR, but during cruise and above a certain... IIRC Power lever angle, N1 was automatically synchronized to avoid cyclic beat.
Actually, if a little thread drift is allowed: With an engine using N1 as power indicator, is there some degradation allowed for when it comes to calculating performance tables?