I can find significant differences taking two tests one after the other - BP changes constantly like BS does. The best time to check is after resting sitting down, legs definitely not crossed, arm outstretched with palm upwards, and not just after a meal. Normally BP will drop after eating as the blood is busy dealing with the digestion, and will rise after/during exercise, even mild exercise like walking upstairs, or if a worrying thought has entered your head.
If you want to get the best out of it, take it at different times of the day and evening and after different events. Then average them.