It’s because there is no scale on Form3 to measure the tank’s weight, so all calculations are approximate.
@Jesse_K
A possible solution is for them to implement a manual scaling menu. You measure the weight of the tank and then enter that value into preform. The program would have a database of the weight of all their resin tanks and calculate the approximate resin left by the weight difference. Or let you manually enter the weight of the new tank upon unboxing.