Diagnosis
You can't calibrate the built-in hygrometer. It's a basic sensor affected by its placement—right next to the moist, warm air exhaust.
How to Fix It
Ignore its absolute number. Use it only for observing relative changes. To get a true room reading, place your standalone hygrometer in the path of the air intake (the sides with filters), not near the exhaust vent on top.
If That Didn't Work
The sensor can be fouled by dust. Power off, remove the filters, and gently vacuum the interior area behind the filter slot where air is drawn in, which is near the sensor.
Heads Up
Set your target humidity based on comfort (like 50%) using the unit's controls, but verify the room's condition with your own trusted meter. The unit's display is best used as an on/off indicator, not a precision instrument.