% speeds for engine rotors are just numbers.
The manufacturer very carefully establishes limits in "real" rpm during testing/certification. When it comes to production, they often just look up the catalogue and buy in a tacho system which will do the job, then calibrate it to establish the operating limits as will be presented to the crews.
Really it matters little if the limits are calibrated in bananas as long as all the documentation is consistent. Think also of derivative engines e.g. GE CF6 series - those on our B744s have N1 117.5 and N2 112.5 as limits, likely because the engine got more grunt and there was no real need to certify a new tacho system.