Because of physiological reasons, blood pressure is no constant body value. It could vary up to 30 mmHg within a short amount of time. That is the reason for different results. It's not easy to always guarantee the same conditions for every consecutive measurement process. It's not unusual for consecutive measurement processes to be disturbed by unstable physiological conditions and influencing additional factors (talking, noises or movements).
That's why the average of mostly all the results taken under the same conditions represents your individual blood pressure. For example measure your blood pressure on three consecutive days, one hour after dinner and you won't notice any remarkable differencies. The average of the results represents yor blood pressure best.