Hysteresis, one more point
I looked up Hysteresis to further clarify points already made. Hysteresis means "delay", in this case, delaying pacing to maximize the hearts own ability to beat.
For instance, if the lower rate is set to 75, then the hysteresis rate would be 65 or 70 to allow the heart to take over functioning on its own, if it can.
Hope that clarifies the issue further.