Diagnosis
The built-in hygrometer sensor isn't highly accurate and can be affected by its own operation. It's a common complaint—they're often off by 5-10%.
How to Fix It
You can't calibrate it through software. The best you can do is help it get a better reading. Move any separate hygrometer to the same location as the dehumidifier's air intake (the front grille) and let both sit for 24 hours. Note the difference. Use that offset mentally when setting your target humidity. For example, if it reads 60% when your trusted meter says 55%, set the target to 50% to achieve ~55%.
If That Didn't Work
The sensor can be fouled by dust. Try cleaning the filter thoroughly (see other fix) as a clogged filter affects the air sample the sensor reads. Also, don't place the unit right against a wall; give it at least 8 inches of clearance for proper airflow across the sensor.
Heads Up
Don't chase perfection with the reading. The important thing is whether the room feels dry and the unit is collecting water. The number on the display is more of a relative guide than an absolute measurement.