Some comments: ISO stipulates documentation developed by the company seeking certification, must follow a standardized order, but not the methodology used. Further, the quality of the underlying company is only as good as the registrar that audits the company to determine compliance to the standard. A company can be ISO registered and not have any procedure in place that describes use of hardness testing, let alone specifying hardness scales, or selection of proper equipment to the material to be tested.
As far as hardness testing, variability in readings can happen and again is determined by the consistency of the operator's methods as well as the equipment itself (not to mention proper use).
I am not a QC tech/QC manager/Director of Quality or any other title associated with the quality department in the manufacturing plant I own, though we do have a dedicated quality lab, with 4 employees administering our quality program and directives (in excess of 70% of our sales output goes into new cars).
If the thunder don't get you, the lightning will.