Everyone is a little different on ox and pulse rate, and some of the meters can give false readings, mine was compared to the really expensive ones in the Dr. office and mine reads 2-3 points high on ox, and I paid $80 for mine. They said that is what most do, but most are very accurate on pulse rate. Like anything medical, it can get a little complex when other factors are involved. Heart/lung function can get real involved. In my case my sleep apnea, and weight, not active enough.
Reduce it to the factors we know we can understand, for me every pound I get off is a pound less load for lungs that are working at less capacity. The PT trains your mind to breathe better, be aware of you inhaling and exhaling rate, we just do not think about. Also learn techniques to recover your ox and heart rates, when you have a drop in ox. The big 3 that are factors are eating, drinking, and smoking, plus inactivity. Luckily smoking and drinking are not factors for me, but we live in smog, and been around industrial bad things.
I'm pretty sure the majority of my friends in PT had smoking as a major factor. I would strongly consider quitting, because eventually it will take its toll on the lungs/heart function.