I don't have a very low resting rate either, about 65 BPM first thing in the morning. It can drop to about 58 BPM when I'm riding a lot and fit. Sitting quietly in a chair it is usually about 75 BPM. This has often had me puzzled about which figure to use when calculating HRR!
My HR also settles at about 105 BPM after exercise for about an hour, sometimes longer. I've been told that this is not uncommon.
Another thing that I've noticed is that my HR is VERY sensitive to anxiety, stress or excitement. e.g. If I'm excited and really looking forward to a ride I stand around at about 100 BPM. If I'm anxious or stressed then sitting down it is often around 90 BPM.
I do, however, take asthma medication which is well know for messing with your HR so all of the above is probably not that relevant to those without asthma! 😕
One last comment: My heart rate has been this way for decades (I'm 45 years old).