This is purely technical and it is a behavior of SAP product architecture. Certain field’s data in SAP will store into database table using its internal format. The data should be fetched from tables with their internal format only. SAP provided some standard function modules to do this. You can see from the domain value of the field whether it stored using internal format or not based on conversion routine. Conversion routine exists for the material number. See the same for its domain value. If you double click on the conversion routine, it will give the conversion function module used for this.