N1 is only confirmation of power if there are no compressor anti-surge bleed valves fitted.The RB211 and JT9 has them and the GE hasn't.
On the RB211 and P&W JT9 you could have a bleed valve(s) stuck open and although you're achieving the appropriate RPMs the engine would not be producing anything like take-off power - a large percentage of the mass airflow is being dumped overboard.
The only TRUE measure of thrust produced is a measure of the pressure rise measured from the intake and fan/turbine exhaust.
Because of this, if you're dispatching with an EPR gauge u/s, you try to reduce the potential problem by not using graduated power and sometimes imposing an RTOW penalty