Why are there errors when using a digital Rockwell hardness tester?

Taiwan Nakazawa Co., Ltd. answers this question for you. Different surface finishes will exhibit different effects during Rockwell hardness testing.
For surfaces with low surface finish, the hardness will appear relatively high during high-hardness testing; conversely, the hardness will appear relatively low. A rough surface with tool marks, when cooled during quenching, or a relatively hard surface layer will result in a higher hardness value. On the contrary, during high-temperature tempering of quenched and tempered parts, the surface structure with tool marks transforms first, leading to lower tempering resistance and thus a lower hardness value. When testing parts with a surface finish below Δ7, it is necessary to use a discarded grinding wheel for fine grinding, then file it smooth with a file, or polish it with a fine hand-held grinding wheel, and then wipe it clean.