In all likelihood your car shuts off fuel flow when the throttle is released above a certain RPM threshold.
The
ScanGauge doesn't actually monitor fuel flow. That's not part of the standardized info available in OBD-II.
So in order for it to "see" deceleration fuel cut-off, it is probably looking at a combination of factors, like throttle position, engine RPM, and loop status.
I seem to recall that there's a Throttle Position Sensor (TPS) related threshold that you can set - maybe that will fix the problem. I've never had to do this with a
SG, but others have and hopefully someone will chime in. I did a quick search with no luck.