In my experience, the accuracy of the Charge 2 HR data is pretty good when at rest (i.e., it matches what I get when I simultaneously take my pulse to within a couple of bpm). When moving about, and certainly when exercising, there can be some issues. I think that part of the problem is that what tracker is recording isn't necessarily what it shows on the readout at any given moment; sometimes, there seems to be a lag between when you know your heart rate is rising (e.g., walking up stairs) and when that gets reflected on the display. I notice this a lot in workouts; I'll go for a run, periodically checking my heart rate on the Charge 2, and never see a number above, say, 158, but when I look at the activity record, the maximum heart rate recorded is 163 or something. So the data are there, you just don't necessarily see them when you happen to look.
I've read a couple of reviews comparing the Fitbit HR tech to others, and if I recall correctly, the wrist-based monitors are generally a little less accurate than the chest straps (no surprise there), but that the Fitbit consistently did worse - up to 10 bpm too low during exercise. Now, a couple of caveats: first, this was a few years ago, and the tech may have improved; and second, they only tested a few units on a few people. I've been having terrible issues with my chest strap HR monitor for months now, so unfortunately I can't say more than that I suspect that the Fitbit does indeed register a little lower than your actual HR during exercise, but I'd be surprised if it were 10 bpm off.
Oh, and as to the numbers you quote - yeah, like the other respondent said, that 72 bpm number is the average, but for humans, the average is completely and totally useless! This is because heart rate varies based on age, sex, temperature, whether you're sick, etc. So, doctors and other professionals are less concerned about whether you're near the average, more about whether you fall within the range expected for healthy adults. I've seen different endpoints used, but usually the normal range is quoted as something like 65-90 bpm (I've seen as low as 60 and as high as 100). Keep in mind that's resting heart rate, and it refers to people who are at most moderately active; athletes can have much lower resting heart rates and still be considered normal (mine fluctuates from about 55-60, and a friend of mine is consistently around 48!).
