No, as far as I am concerned. But I agree that different batches of the same steel type (304) could explain this different magnetic behaviour. There is another explanation possible. This screws are processed by cold stamping techniques and this deformation transforms partially 304 austenitic steel into martensitic material, which is magnetic. The combination between batch material properties and cold stamp deformation effects could cause these different magnetic properties. Although this possible reasons, I really don't have a 100% sure explanation.