Now I know ... My case humidity levels are different.

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My guess is that the hygrometer in the neck area has the benefit of greater area from which to draw its readings. The unit in the rear looks to be pretty cooped up when the lid is closed. I would be interested in seeing a similar test with the tail hygrometer placed in the soundhole where there is room for more air volume/movement.

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Moisture needs to be able to "flow" from the humidifier to all areas where the wood and case components might be absorbing it. If there is a pad between the headstock where the humidifier is and the body, this will impede that flow. Good idea to put other humidifier in the sound hole.