This sounds like a question to ask Ron at Linear Logic. I've always found him to be very responsive to specific questions.
I've also seen this behaviour. On a recent trip with the car warmed up, flat roads, negligible wind, approx 45-50 mph, I watched the SG "instant" screen toggling exclusively between 66 mpg and 83 mpg (no intermediate values), and I could influence which I saw with an imperceptible movement of the throttle.
smartzuuk 
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