The engine is flat rated at 26K or 27K (or lesser ratings) according to the customer's requirements. These are in turn determined by the airline's route structure. If less thrust is required, then the engine deterioration rate is reduced, and the maintenance cost per hour is predictably less.
But the engine is flat rated up to a specified ambient temperature (commonly ISA+15C), and the rated N1 to achieve thrust depends upon the ambient temp and pressure.
In addition, more thrust (N1) can be had if bleed air is reduced during TO. This is because EGT will be reduced at a given N1; thus the bleed data is included in the computation.
To get an idea of how N1 behaves under different conditions, see the N1 chart in the performance section of the AFM, or find a copy of the CFM Operating Instructions.