Pablo HM

If we start with a base of 715 kWh/m2 incident on ground and we apply all losses and gains that appears in the Sankey diagram attached, we obtain an effective Global Irradiance on rear side on collectors of 60,96 kWh/m2. There is a lot of discrepancy between both results. If we do the same calculation for the front side, we don't have any problem. Why is this happening? Do we have to consider anything else as configuration 1P, 2P... or other variables?