My guess is that when you start driving, it starts looking for moments of maximum longitudinal and lateral accelerations and sets that as a baseline. The maximum then ratchets up if it senses periods of greater acceleration. Not sure how it would adjust to deteriorating grip due to tires, rain, brake fade, or track conditions.
According to one of their videos, the thing is really trying to compute a grip profile to create a traction circle. (See Driver 61 University Series on You Tube for explanation of a traction circle if you don't know what it is.) The display then reflects how well you drive to that circle. The circle is a goal, not necessarily a perfect reflection of reality, so maybe if the prediction is in error (presumably not by much), that wouldn't negate the utility of the device.