I think it might be due to the flat terrain you ride on as you're a super efficient rider on the flats.
I'm similar to you as I'm age 59 and have a resting BPM of around 48 (according to my Fitbit). I ride solo 5-6 times per week. Each ride is around 19-22 miles with about 1800 feet of climbing and my average MPH is 18-19. My average BPM is always around 130 but maxes at around 180 when I'm doing the steep climbs. I'm also a lightweight as I weigh 130 lbs so usually do 95% of my climbs in the saddle. I ran track in high school, biked in my 20s, started running again in my 30s and 40s when I had kids, then started riding again when I turned 50. I also swim at the beach for around 25 mins during the summer months.