We are reading too much into the "tests" done for Rockwell C scale hardness.
One sample posted reading on a spreadsheet with several contributors does not tell the full story.
There is no reference to manufacturer/model of hardness tester used or calibration procedure and maintenance schedule.
For example, the photo above with the Wilson tester may indicate a very inaccurate reading. Poor operator technique is obvious,, the sample is not supported fully by the anvil,,, the knife is hung over the side of the support anvil and will most likely give a false reading. This is clearly explained in the operator's manual.
https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication960-5.pdf
A more accurate reading should have the knife blade disassembled to minimize the over hung load potential variation.
The older Wilson testers were well known for lack of repeatability and reproducibility if not disassembled, cleaned, lubricated and calibrated frequently. Newer designs have load cells and digital readouts to improve accuracy and eliminate the mechanical dead weights and other friction parts.
Regards,
FK