It looks like the sensor for the humidifier is on the back wall. The RH there is different than the RH for the plant. I always hang the sensor so that it's near the plant.
Unless you flip them very young, two well tended plants will fill a 2' x 3'.
This is one "well tended" plant in a 2' x 4'. The sensor is moved out of the way for the photo.
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Note the sensor in the foreground.
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Re which sensor - the sensor in the AC Infinity humidifier is most likely the more accurate.
In addition to the AC Infinity sensor, I've used el cheapos, an AcuRite Pro Accuracy hygrometer, an Inkbird RH controller, a Pulse One, and a Pulse Zero. When I calibrate them (salt test + a 75% Boveda calibration "kit") the AC Infinity was accurate out of the box to a few tenths of a percent,