My husband tried several different home blood pressure monitors, one after the other. The last one he took to the doctor to have it calibrated against the doctor's BP machine. It was 30 points off. So he took it back and never got another one. Now he and I both check our blood pressures at the local pharmacy. Incidentally, I lowered my blood pressure 14 points with three cups of hibiscus tea (took 3 weeks or so to take hold): see the National Institutes of Health research report on this. I didn't believe the research could possibly be right until I tried the tea treatment myself! It works! The higher your blood pressure to start with, the greater the benefit. If your blood pressure is close to normal, it won't change much with the tea, says the NIH research report.
It appears you take your blood pressure at home, if so make sure that your blood pressure cuff gives a similar reading to the cuff in your doctor's office. Electronic blood pressure cuffs need to be checked periodically, to ensure that they are giving an accurate reading. A malfunction BP unit can cause variations.
What is your heart rate when you BP is high, and what is the heart rate when BP is low? Irregular heart rhythm changes - if your heart is beating irregularly, it potentially causes your blood pressure to be low as a result. The blood is not pumping through your body very effectively, and your body is unable to compensate.
You should monitor your blood pressure at the same time every day if possible, while you are doing the same activity. The morning is the recommended time before beginning the day, medications can also throw off your blood pressure, so if you take pills first thing in the morning, measure your blood pressure before that.
variations of 30 or even 40 mm Hg differences during a day are not uncommon. If greater you should inform your doctor.
Hope this provides some insight, and if you have any further questions or comments you are invited to respond. Thanks for the question, take care,
Ken