When I worked in the shop test cells, where you get the true static thrust reading, working RB211's, with the FADEC control. You would see at least a 1% diff, if not more in N1 speed, between engines at the same take-off and max-continuous thrust. This demostrated that each fan was individual in it's performance, and speed required to attain the true thrust. Yes, EPR is also a 'moveable feast', but RR believe it less of a moveable feast than N1.
If you were running to an N1 and the efficiency of the fan meant you were 1% or more lower on actual thrust, then it may only be 600lbs.
BUT, remember that all these engines have a 'trimmer' system, be it EPR/N1, bringing them closer than 600lbs to the true thrust.
Brgd's
N1V