Gas turbine aero will tell you this:
If in fact EPR = 2.0 on both engines, and the -17 produces more thrust, then the exhaust nozzle area of the -17 must be greater than the -15 (i.e. more mass airflow through the nozzle)
And to pump the greater mass airflow, more fuel will be burned.
On the other hand, if the two nozzles are identical, then the EPR's are not exactly the same (even though the calibration of the gages is "trimmed" so they look the same).