Several people have asked me this. The most likely answer is that people in warmer climates preferred to take readings in the morning, and people in colder climates preferred to take readings in the afternoon.
Disrupting the Borg is expensive and time consuming!
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Real Science 
The interesting thing is: There is a 2 deg F difference. So given the magnitude of the delta how can various outfits report temperature difference of 2/10 of a deg?
Suppose the morning stations are further south on average?
Morning, midnight, afternoon or evening, it doesn’t matter. There is nothing in the world that can ever take away from 1936! Like average high temperatures during July 1936 being 20 degrees hotter than 2014.
Somebody got the colours reversed, or it there was a shortage of mad dogs and Englishmen willing to go out in the afternoon sun?