hi
it's quite natural
you cannot use the ADC for it's full scale
once i tried and could only achieve 20 bits with last bit toggling.
it depends on many parameters. so even if we get 16-18 bits without any problem, it's OK.
the PCB routing, impedance, capacitance, resistance of tracks, and other components,grounding like many more parameters are there.
even if it's 16 bit, 65536 counts are not bad.
i don't think you are going to design a class 1 weighing machine.
for class 2 and 3 16 bit resolution is quite good
regards
ml