Two answers are given in the following.

1)  One possible explanation to why different temperature responses could result from two identical tests could be a  disturbance.  Typical disturbances for the heat transfer experiment would be the inlet temperature, upstream pressure of the heating medium, and feed flow rate.   These disturbances would cause a discrepancy in the results of the two experiments.

In order to avoid unmeasured disturbances, the personnel performing the experiments ensure that all of the input process variables that could influence the output remain essentially unchanged during the experiment.

2)  Another possible cause of discrepancies would be a sticky valve that did not move the amount expected.  To avoid this error, we should monitor the actual valve position to ensure that the stem moves the amount desired.