You have not said if you built this plane. Assuming you did not . . the dihedral of the wing means the fuel tank fills to the top on the inboard end first. This means the short arm of the float (assuming you have a float) reaches it's upper limit before the tank is filled to 21 gal.
When the G3X fuel levels are calibrated, one must trick the system to indicate higher than the maximum fuel actually measured. My 7 float on the right tank will top out at about 17 gal when filling. Once the tank is full, the gage will not indicate any change until it reaches 17, then will follow the calibration curve. I did not want a full tank to indicate 17, so a 1/2 gal short of 17 was calibrated at 16.5, but at max height (17 gal) I entered 21 gal in the calibration curve.
Does this make sense for your situation?