Or is it like boxing and there are multiple governing bodies and Tuesday captured the WBO hottest day, but the IBF and WBC still recognize August 1, 2022?
Generally in the US “official” numbers would come from the NOAA and the NCEI would be the one publishing them.
How do you ideally compute this average? You take the simultaneous temperature measurement at every point on Earth and take the average. Obviously that is not possible.
What do you do instead? You put together temperature readings where they exist, satellite data, coupled with past data and behaviors to make an estimate for the average temperature. How else could you possibly do it.
[1] https://docs.house.gov/meetings/SY/SY00/20160202/104399/HHRG...