I don't have a power meter and use HR and RPE to judge what is going on. I find results from this to vary to a surprising degree and sometimes varying results seem to have no (discernible) reason for being. For example .....
My 2'ish hour ride last 8 days ago vs. yesterday. These were the identical route, similar temps, similar wind conditions (not much), and same biker/bike. RPE was (to my way of judging) identical. Also the total weekly workload in the two weeks were similar (and typical for me) and the workout the day before each ride was pretty much the same. On both days I felt "OK but not fresh".
Last week I averaged 17.8 mph, average HR of 149, and max of 161. Keep in mind that I am old (65), my max HR is around 170, and LTHR is around 157. My perception of this effort was 'strong' but there was clearly something left in the tank at the end.
Yesterday I averaged 18.3 mph, average HR of 143, and max of 158. My perception of this effort was 'strong' but there was clearly something left in that tank at the end.
0.5 mph is a pretty meaningful change in speed (IMHO). But an average of 6 bpm is a HUGE change in effort for me (on a given day) when you are operating at over 90% of LTHR. Yet I rode this non-trivial amount faster at (per HRM measurements) a much lower effort (although RPE was the same as best as I can judge).
I suspect that this is common but I'm curious as to whether other HRM/non power meter folks experience similar results.