In the hit the hammer penetrates into the roll just about 0,1 mm (100 microns) or so depending on a hardness of the material. But the effective measurement depth is much higher. When you have a wound roll of thin material, like thin paper or plastic film. You have hundreds or even thousands of layers in the roll.
If there is profile variation in the product, very small variations will add up huge results because of many layers in the roll. So, basically all layers in the roll have an effect on hardness measurement from the surface. In attached article the phenomenon is well explained.