To be honest, I'm having a rather difficult time following your request. What do you mean by "average"? The watch doesn't show any average heart rate anywhere outside of a workout.
If you mean the resting heart rate indicated by a blue dot in the (in mine, and a lot of people's eyes, useless) min-max-graphs, that's just a value (of which I have actually no idea when or how exactly it takes them) that is supposed to show your resting heart rate on that given day. Usually, any fitness tracker or smartwatch with heart rate measurement actually takes the value in the time between you waking up and getting out of bed, or shortly before going to sleep while lying in bed, and the resting heart rate is an average of that. But on the Samsung wearables it's apparently not an average, and to my knowledge, it only shows up with the continuous measurement turned on. The measurement every ten minutes only takes place when you're moving very little anyway and it only measures "while still", and thus "resting".
But it doesn't show any "averages", and the ten-minute-setting doesn't show the daily resting heart rate that you're probably looking for.
As to why it sometimes doesn't show it even with continuous measurement, my guess is that it sometimes doesn't show it because it assumed that you never really rested, or because the continuous measurement got interrupted long enough on a given day (as in: It shows unreliable or no results because, like a lot of people, you might be wearing it way too loose, or it just doesn't get worn all day).
I assume that nobody will be capable of precisely telling you about the certain measurements or values besides the developers themselves, who are probably not on this forum.
