I can't give you an opinion on the different figures you have experienced between constant speed and fixed.
I do have an opinion on POH figures in general. The AC I fly now is fairly new (constant speed) and the POH figures are pretty close.
Other older AC I have flown don't get near the POH figures.
I think it is to do with age. Not so much the aircraft as the manual. There has been a change in culture. A few years ago the POH was being used as a sales aid. The figures nowadays are a lot more truthful.
D.O.