Variations in weight. Cases that weigh more have different dimensions than cases that weigh less. This results in identical powder charges developing different pressures, which causes bullets to fly hither and yon. The really serious F-class competitors I know will take a batch of 100 shells, weigh them, and segregate them into three groups, light, medium, and heavy. Each batch of shells will shoot to a slightly different point of impact, but each batch will group together, so you can compensate by shifting your point of aim.