So, I bought a second tester and triggered them simultaneously then I waited 10 min, switched the
testers from one arm to the other and did it again. The differences that I saw then were within 4-5 mm HG, easily explainable by plumbing variations between the two sides. Now, why did your blood pressure spike that much in such a short time? All I know, in my case it happens, but it doesn't occur that often.