Not a typo – this has been called the “Conconi point” although there are so many definitions, it’s hard to keep them straight.

You might see a fairly linear relationship between intensity and heart rate at lower intensities. From 9:30 min/mi to 9:00 min/mi to 8:30 min/mi pace, your heart rate might go from 140bpm to 150 to 160. But but there’s an upper limit on heart rate, so the increase to 8:00 min/mi and 7:30 min/mi might only raise your heart rate to 166 and 172.