A healthy heart is not a metronome

Your heart rate is constantly changing to adapt to what you're doing and keep your body in a steady state (homeostasis). As such, your Heart Rate Variability reflects your body's capacity to adapt to physical and psychological changes.

In general, a higher HRV indicates better fitness, but too much variation means you're wasting energy. Worse even, too little variation indicates chronic health problems, where the body is struggling to adapt to changes.