Why jet engine RPM increases by increasing altitude at fixed Throttle or PLA settings ?

Question

I experience an increase in engine RPM by gaining altitude at fixed Throttle settings “without increasing throttle power lever angle”
For instance at 2000 feet the engine RPM is 96% at full power.. at 13000 feet the RPM is 98% at full power as well. Could someone give me explanation for this phenomena ?

In: Physics

Answer 1

I’m guessing that it’s a reduction in drag due to the thinner air?