Heart rate can vary with physical condition, health/illness, prescription and OTC medications, caffeine, alcohol, many factors.
At age 62 mine is all over the place. My resting and maximum HR runs higher when I take meds for allergies and asthma, lower on other days.
So I tend to look for trends over time, rather than pay much attention to any single workout or even an entire week's rides and workouts.
The last test I did to check my maximum heart rate was summer 2019 when I got a Wahoo Tickr. At the time I could consistently hit 170-173 bpm in tests I repeated over the summer. The last time I hit 171 was on a maximum effort 0.4 mile steep climb, back around October. I doubt I could hit 170 now. I'll repeat a max HR test later this year, after the current COVID-19 pandemic scare is over. My most recent blood work showed elevated monocytes (I had a long term autoimmune disorder with chronic inflammation), so for now I'm avoiding HIIT and max effort workouts.
Some rides, during the first 30 minutes, my HR hits 130 bpm just coasting down the slight hill from my parking lot and pegs around 160-170 bpm on the first hill. After an hour it settles down and stays around 130 for consistent moderate efforts and hits 160+ only on maximum efforts.
But this past week I couldn't get my heart rate above 140 bpm, even on efforts that usually bump it to 160+. My usual resting pulse is around 65, but it's been as low as 58 the past few days, and my BP was as low as 104/48 (I usually run around 120/60). Probably due to a muscle relaxer I take for neck and shoulder spasms from injuries. But it seemed to persist more than 24 hours. I don't take that med often, less than once a month, so maybe that's just how long it takes to clear my system.
And my endocrinologist just increased my thyroid meds, so I'll probably see some increase in BP and HR when it takes effect. Then it'll probably settle down again after a few weeks.
Another factor for me is riding style. For years I was a spinner, around 90 rpm, even before it was fashionable. Just felt natural. But last year I noticed my heart rate pegged way too early on climbs and I'd gas out suddenly. After getting the Tickr I had enough data to confirm that impression.
I went back to 1990s articles and re-read everything I could find about the reasons for the transition from the old days of slow mashing efforts (it wasn't unusual for pros through the Merckx era to grind as slowly as the mid-40s to low 50s on long climbs). The faster spinning style, practiced by many but mostly attributed to Lance Armstrong under Dr. Ferrari's tutelage, was intended to reduce wear and tear on the legs, which recovered slower than aerobic efforts. A top athlete, trained and accustomed to spinning, would still be in pain from a burning chest, but could recover overnight from that. But cooked legs could take a day or longer to recover. (And, of course, EPO and blood doping helped with the transition to the heavier aerobic demands of spinning.)
Basically, it made no sense for me to emulate a 100-110 rpm spinning style. I'm not racing up to 200 miles a day for three weeks at a time with only a couple of days rest. I'm riding 3-5 times a week, 20-50 miles per ride.
So around August 2019 I deliberately changed pedaling style in favor of a slower cadence and harder gears. Coincidentally I was also riding a new-to-me 1993 Trek 5900 that had been fitted with old school Biopace 52/42 eccentric chainrings. Reportedly those worked better for some cyclists at 80 rpm or slower.
It took a couple of weeks to actually get comfortable with mashing harder gears -- I didn't realize how lazy and poorly toned my legs were. But pretty soon I was comfortable at 60-75 rpm. And, sure enough, my heart rate dropped overall on the same routes, same speed, same perceived efforts (allowing for wind conditions, etc.). I got stronger and faster for the same effort.
So I'm still mostly mashing, although occasionally I'll switch to my normal 90 rpm (it still feels natural to me) just for a change of pace, and to relieve any slight aching in my legs and knees during a ride. Around 5 minutes at a faster cadence in an easier gear is usually enough to recover and go back to slower mashing. And I can see the change in heart rate corresponding with my cadence change.
But that's just me. Dunno if that variation in cadence and gearing would make much difference in heart rate and overall performance for other folks. It'd be interesting to hear from other cyclists who've tried a methodical change in cadence.