(NB: All this assumes we're talking about a fixed-pitch prop with a non-turbo engine)
The important thing to understand is that when the POH talks about "percentage power", they mean a percentage of the engine's maximum rated power. In a non-turbo engine, this power is only achievable at sea level (and even then you'd need to be moving at high airspeed otherwise you won't get the RPM you need to achieve that power).
So, in practice a firewalled throttle will never give you 100% power. The reduced air pressure at flying altitudes will reduce the power output of the engine, so that at 5000 feet (for example), it's probably only possible to get about 75% power at full throttle. The power tables in your POH will give full details.
The other thing, as others have mentioned, is that the relationship between power and RPM is very non-linear. At higher RPM, a small change of RPM corresponds to a significant change of power, whereas at low RPM you need to make larger RPM changes to get the same change of power.