The short answer is that HRV is not a static measure and fluctuates from moment to moment, much like blood pressure. The normal variation of HRV when the measurement is done properly is 2 HRV points on the BioForce scale and ranges greater than this typically represent outside influences affecting the results. The vast majority of the time, the first HRV test will provide the most accurate reading and there is no reason or benefit to doing multiple tests.