It’s not the variability in average heart rate, it’s heart rate variability. It’s the amount of time between beats. You can have two people with an average heart beat of 60 bpm. The first has low variability, consistently one beat per second. The second with high variability: just under a beat per second, then one beat over a second later, and so on; averaging 60 bpm.
Low variability means a more consistent rate.