If the readings are always low, then the reference level point and the bottom level points are suspect.
If your zero level is configured correctly, then a long shot is speed of sound effects: temperature or vapor composition/density. But I think this is a setup/configuration issue.
Is the error constant? How big is the error?
What are the level points used?
Is the tank pressurized? Vapor blanket? What is the temperature inside the tank?
The reading is low it means the range values went wrong..so it is better to calibrate both zero and span trim ...
And clean the sensor before calibrating..