How is solar thermal panel heating calculated?

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I started looking more closely at the SolarCollectorFlatPlateWater.idf example file and the Engineering Reference for the Solar Thermal panels.

I found these rows in the output of the example file output variables.

The two rows have approximately the same outdoor temperature, water inlet temperature, and incident solar radiation, but a 2C versus 8C jump in the water temperature. I thought heat absorbed by the water in the panel was only dependent on the solar radiation, outdoor air temperature, and water inlet temperature. What other variable influences the performance of the solar thermal panels?