Heat Index Calcuation and Value differences

I have both a Tempest and Davis Vantage Pro 2 mounted on the same pole. During the period of extreme heat we’ve been having here in Oklahoma, I’ve noticed a heat index diffference between the 2 of as much as 5 degrees even though the temps and outdoor humidity readings have been closer in their compartive values. Any ideas why there would be such a big discrepancy between the 2 stations? I thought there was a standard formula that is used to calculate the heat index.

I don’t know about the Davis, but the formula used by weatherflow can be found here: Derived Metric Formulas

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Davis formula seems to be in http://climetua.fis.ua.pt/legacy/main/current_monitor/WLinkExpVar.pdf which has some interesting reading re: ‘old’ and ‘new’ formula which is a little surprising.

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