Blood pressure is not a fixed number; it rises and falls constantly with your heartbeat, breathing, activity, emotions, time of day, temperature and even talking or a full bladder. It is usually lower during sleep and higher in the morning and during stress or exercise. So it is completely normal for readings minutes apart to differ.
Measurement technique also matters. Wrong cuff size, an unsupported arm, crossed legs, talking, or not resting first can all change the result. This is why a diagnosis is based on the average of multiple readings on different occasions, not a single check.
To get reliable readings, sit quietly for five minutes, keep your arm supported at heart level, feet flat, and take two or three readings a minute apart. If your readings swing very widely or are often high, bring your log to a doctor or pharmacist who can check your monitor and technique.