Interesting questions. Although at a population level there are obvious patterns, at an individual level there is too much inherent variability. As an example, I included a figure from Whyte et al. (Int J Sports Med. 2007) showing max HR data for elite (Olympic level) athletes. Age shows up nicely, but at any given age the spread is pretty big. For that limited population group then, genetics must play a significant role in the variation. To exemplify further the inherent genetic component: I have a long history of endurance training and a normal week has long been 8-12 hours. RHR is typically 40-44 and max is <170 (clearly age related, but was never high to begin with) – not unusual values for lifelong runners, though on the lower end of the range. A 3-yr older sibling who trains maybe 25% of what I do, has an RHR still <50. Clearly an underlying genetic factor to our low RHRs, but with training history separating us.
So besides training history, there are age, specific phase of training (increased fitness lowers max HR), and genetics amongst other factors. As for inferences of comparing HRs between individuals, it is of course interesting, but not meaningful. An ultra running colleague and I once compared our respective paces required to hit the theoretical MAF HR (180–age+5) – I had to run >2 min/km faster than he did (done on flat bike path). He had to jog to stay below his value but I had to do a tempo run to get up to my theoretical value (and into zone 3 based on both RPE and testing).