Discrepancy between calculated TSI in solar_radiation.py and ERA5 TISR

Hello,
I hope this message finds you well. While working with the `graphcast/solar_radiation.py` script, there appears to be a difference between the global Total Solar Irradiance (TSI) calculated by `graphcast/solar_radiation.py` and the Top of Atmosphere Incident Solar Radiation (TISR) from ERA5 reanalysis data.

On January 1, 2007, at 13:00 UTC, I compared the following:
- Global TSI calculated by graphcast/solar_radiation.py
- TISR value from ERA5 for the corresponding moment
- The difference between these two values

Is this discrepancy expected due to differences in calculation methods or data sources? Or could there potentially be an error or inaccuracy in the calculation method used in `solar_radiation.py`?

Start with graphcast/solar_radiation.py and reproduce the comparison for January 1, 2007, at 13:00 UTC using the reported ERA5 TISR value. Trace the inputs and calculation assumptions, then determine whether the discrepancy is expected or indicates an error and document the finding or required correction.

Written by the indexing model from the issue text.