The OwnIndex is also based on activity level. You should set your activity level to the appropriate ones as you are currently participating at, which has not changed for the past 6 months. There is Low, Medium, High and Top.
From High and Top, it varies a lot, whereas the Low, Medium and High does not vary too much for me. Polar might have a certain formula, but it's formula is dependent on the R-R variability or something. It measures the interval between each heart beat, where the greater, the better the physical state you are in.
I am not exactly sure as to what is really means, is it the interval between each heartbeat or the variability of the interval between each heart beat, which would be like 1.0, 0.8, 1.3, 0.7, 1.2 for a 60bpm resting heart rate. Can someone clarify?