Yes, all these guidelines for taking BP may be true, but most doctors' offices violate all of them, and most doctor's offices will only accept their own, incorrectly taken, BP readings. And base their prescriptions on their own readings.
So it makes you wonder motives, doesn't it? Why is my BP always lower at the blood center? It it intentional so they can get more "pass" tests? I doubt it, but makes you wonder.
Conversely, it is to the advantage of PCP to get you a "fail" so you get on the medicine and follow up visit never-ending treadmill?
I think in both cases I'm projecting worst case motivations. It is just a matter of timing and circumstances.
My doctor takes my BP with me sitting on the uncomfortable examining table and him usually talking to me. Talking is a no-no, and I find the posture uncomfortable. Still, he almost always gets a 120/80 from me.
Meanwhile, at the blood center, I am sitting in a nice chair with feet flat on the floor with my arm resting on the desk at heart level, and I frequently read something like 108/69 there.
Is it the machine? Is it the fact the blood center follows all the guidance?
Meanwhile, my dentist suddenly threw a BP test at me as a "surprise." It was a weird machine they put on my wrist and then made me move my arm to my chest which randomly seemed to activate it. It was mysterious to me and unexpected. It caused some anxiety even wondering how the machine works (what a weird machine). It called out a 140/80 on me. In the meantime, in the other room, I hear a guy complaining that his BP is never that high.