The standard derivations of L/D in the glide and speed for best range, best climb etc all assume:
- simple drag polar Cd = Cdo + K. Cl^2
- constant propeller efficiency and constant specific fuel consumption
and ....
The drag polar has a break in it - above a certain value of Cl, drag increases by more than K. Cl^2. There are slipstream effects on drag and .....
Main reasons:
- the characteristics of a fixed pitch propeller are very much different than assumed above.
- SFC is not constant
Hope that answers the question that you intended:
"why is speed for maximum range different from speed for best glide angle?"