The oximeters used by my pulmonary rehab program, very high tech, are always all over the place and obviously inaccurate. I have a basic one at home that we must have bought years ago from the drug store that seems more reliable. Also keep in mind, according to my pulmonary rehab therapists, if hands are cold will impact readings, nail polish, movement both also impact readings. Sometimes it can just be about fit. For heart rate, I use a Polar chest strap monitor which is very accurate (I have a low resting HR, about 50-55, and take it up above 80% max, 140-155, during exercise). My oxygen levels are still pretty good (95-98%), so I haven’t spent time researching more reliable oximeters but if/when I do, I will report back anything interesting.