Biology Medical Journal (Univesity Level)

The passage is taken from 'Comparison of the Atkins, Zone, Ornish, and LEARN Diets for Change in Weight and Related Risk Factors Among Overweight Premenopausal Women'.
It mentions that 'resting blood pressure was measured 3 times at 2-minute intervals as described elsewhere; the initial reading was discarded and the last 2 readings were averaged.'
Please give reasons why the initial reading needs to be discarded.