simulation of the bird droppings shading

[Chen]

Dear sir：

Recently, when simulating cell-level shading of PVsyst 8.0.19, 

I placed the shading object about 1-2 cm away from the PV module surface to simulate the bird droppings shading. 

Simulation results showed that the percentage of electrical shading loss was almost zero, 

Yet the 3D modeling confirmed that shadows were cast on the module surface. What might be the cause of this? 

Thanks！