Thrust has a great deal to do with fan flow capacity for the big iron, which is critically tied to the fan blade tip condition and clearances over the blading in the engine. All these vary from engine to engine, so even with a fixed amount of RPM the thrust can vary.
There are also variances associated with the control logic on the engine. If the control is able to meter more fuel than the internal pressure goes up along with temp and coresponding RPM, if the blade flow doesn't break down in a stall, than more thrust will be produced.
Large margins protect you from the damages of centrifugal overstress, but if you get too high above 100% RPM you may encounter hidden vibratory stresses within the rotating components.