# 1 it is prop pitch that you are changing.
#2 when you reduce prop pitch from default, you are feathering the blade, so it takes a smaller slice.
We have constant speed props, so when you lower rpm, prop wants to spin faster, so it lowers manifold to compensate.
There are guys that do nothing but debate this stuff all day long. Go search for threads about props. You'll find tons to read.
It is the way it is, and its as close as we are going to get for the time being to reality. Pull up your E6B in flight, and see the results of the different settings.