In other words, if the quality of pig iron used by the iron foundry is better and the shrinkage rate is smaller, then the density of ductile iron will be higher. Otherwise, the density of ductile iron will be lower. Therefore, it ranges from 7.1 to 7.3 g/cm3.
Iron foundries usually estimate the weight of rough castings by calculating the volume of ductile iron, so the density will be used. However, you cannot judge the quality of castings by density alone.
There are more factors that will affect it, such as spheroidization rate and cooling rate. Hence, it will be reasonable within this range.