Those machines never read as accurately as a well trained physician with a sphygnomanometer. Of course you can’t find a an actual by God well trained GP with a geiger couneter.
The point is somewhat not one of accuracy. It’s getting your home device verified as measuring what the doctor’s office device measures.
If you are then seeing much lower numbers at home, regardless of what they are those are the numbers that you should insist be used. A nurse will watch as you verify your device matching their device so there won’t be any question from the doctor about that and most devices now for at home retain memory. So you can cycle back through memory to the dock and add to his confidence even further