A very simplistic explanation is "100% of the design RPM".
Most likely, the original A model ran at 100%, and with time and experience, it was found that 104% gave better results with an acceptable increase in wear and tear.
In the S76B, we run at 107% - you may ask why the gauge doesn't get recalibrated to read 100%? Forked if I know, maybe too expensive to have different gauges for different machines.
I have always considered that RRPM in percent is easier to cope with than the raw numbers.