They aren't accurate. That's why there's a notice next to them saying "not for diagnostic purposes." Why are they inaccurate?
1. The cuff might be the wrong size. These machines have a standard size cuff, and if your arm is too big or too small for the cuff, you'll get an incorrect reading.
2. You put your arm too far into the cuff, or not far enough into the cuff.
3. You tensed up or held your breath (without thinking) while you were in the machine.
4. Blood pressure can vary as much as 20 points within a few minutes. So can heart rate
5. Taking your blood pressure repeatedly can cause inaccurate readingns. The cuff does compress your blood vessels, after all. My students practice on me, and my blood pressure can vary as much as yours does in your description.
Bottom line? Those blood pressure machines can be inaccurate.