The airline has the option of buying any of several thrust ratings. There may be no difference in the engine hardware, but the derated engine will enjoy longer time on wing and lower operating cost because of the lower turbine temperature and reduced rate of deterioration. If his route system has long runways at moderate elevations, he may see no advantage in more thrust. It's an economic decision.
The N1 for a given thrust varies with OAT and pressure altitude, so 100% is a purely arbitrary number. On a given day, TO N1 may be greater or less than 100%.
Usually an operator has the option of buying a rating increase after delivery. There may be many details in this upgrade - reprogrammed engine controls, improved performance charts in the AFM, associated software changes... but it can be done.