In article ,
"BDS" wrote:

Anything that would cause a decrease in
power should also cause a decrease in rpm with a fixed-pitch prop.

Are you certain they are the same pitch?
Just because it is fixed pitch does not mean they are the same pitch.
There are "cruise" props, there are "climb" props and there are props
with their pitch somewhere inbetween.

The club I am in has to C172N's.
One has an intermediate pitched prop, the other has a cruise prop.
Both read the same RPM, but the cruise prop equipped aircraft does not
climb as well as the intermediated pitch equipped aircraft.
In level flight cruise, it is 10 kts faster, however, at 2300 RPM.