Why the Site Diffuse Solar Radiation Rate per Area is smaller than the Surface Outside Face Incident Sky Diffuse Solar Radiation Rate per Area

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I was modeling a shoebox office with two windows facing the south using EnergyPlus, the windows are two vertical plane. However, when I extract the hourly report, I found the Site Direct Solar Radiation Rate per Area is smaller than the Surface Outside Face Incident Sky Diffuse Solar Radiation Rate per Area at some hours. I didn't understand why this happened really.

Below is the picture of the model:

Below is the results: