You are correct, 16HV is not a big difference. This could be due to variance in structure also. Certified hardness calibration block have a acceptable range of +/- 10 HV, depending on the test load and range. We actually accept values down to 501 HV(49.5 HRC). I order to satisfy our confidence, we test a 0.001-0.002 below the last reading to ensure the values are below 500HV.