Your right that epr maintains steady symmetric thrust for t/off and N1 is king in the crz, but EPR is used to set all flight segments on the RR RB211 and trents, so it will symmetrically match epr right across the f or 2 engines resulting in differing n1's on each engine hence the beat in the crz. It is very subtle but when some sod points it out to you, the brain latches onto it and it drives me to despair eps when on the 777 you aren't allowed to take the a/t out, an incredibly poor design for modern jet engines for pax/crew comfort