If it's anything like the BAe146 N1 compensation, it is not "live", it doesn't measure thrust directly. Periodically the engines are checked for performance and their thrust output vs N1 is measured. on the 146 you can see the compensation values when the engines are shut down. The N1 gauges will read small values rather than zero. These are the compensations.
So it is not related to any ambient temps or pressures, it's all to do with how the engine performs under test conditions compared to the nominal engine performance.