Muscle fatigue could also play a role. No glycogen storage in the muscle, lack of capilary density, and plain weakness could lead to earlier muscle fatigue than would be suggested by HRR. If you are doing some exerxise in a specific range, i would use a combo of RPE and HRR in determining intensity. More conditioning should help these numbers even out though.

Side note, RPE could be affected by motivation. If they know that theu can stop at a certain point and they hate exercise, they will reach that point sooner.

Just some ideas from med student with a background in the area.