consideration of optimum inclination angle by parametric simulation

Hello,

I am trying to consider the optimum inclination angle of PV array at many place all over the world.
I used the parametric simulation.
As a result, there is a subtle difference of the optimum angle between the annual energy and input radiation.
For example in Cairo, the optimum inclination angle in terms of the input radiation is 25.1 degree,
however the optimum inclination angle in terms of the annual energy is 25.6 degree.

Where does this difference come from? Module property? Reflection?
This is a small difference but I want to know how SAM calculates it.


Aika.