"Global land surface atmospheric variables from 1718..." dataset has incorrect mean temperatures

Dataset Global land surface atmospheric variables from 1718 to present from comprehensive in-situ observations contains mean temperature values different from arithmetic mean of max an min temperatures.

E.G.

index observation_id report_id data_policy_licence longitude latitude observed_variable observation_value value_significance units quality_flag source_id report_type station_name
15 IT000016090-3-1975-01-06-85-2 IT000016090-3-1975-01-06 1 10867 45383 air_temperature 270.85 2 K 0 166 3 VERONA
16 IT000016090-1-1975-01-06-85-1 IT000016090-1-1975-01-06 1 10867 45383 air_temperature 266.65 1 K 0 229 3 VERONA
17 IT000016090-2-1975-01-06-85-0 IT000016090-2-1975-01-06 1 10867 45383 air_temperature 278.15 0 K 0 232 3 VERONA