An engine will have a limit to the RPMs allowed. This is a limit set by the mechanical stress on the Fan/compressor/turbine blades etc. This is the maximum allowed N1/N2.
An engine will also a have rating eg. 56000 lbs. This rating will not be reached at a fixed N1. The N1 will vary with the atmospheric conditions, such as pressure and temperature.
The lower the temperature the lower the N1 and vice versa.
Although at one point you will reach the egt limits, if the outside air temperature is high enough, and then this will then limit your N1 and your thrust. This is also known as a flat-rating temperature. Up to this temperature you will have the rated thrust available.
The percentage used by the various engine manufacturers can have various origins. It could be that the 100% is where the engine is most efficient, or it could be that it really is the max N1 or it could be that it is just an arbitrary value picked during the design process.