its either proper amount of fuel or NO fuel
LESS fuel then the engine needs = running uber lean, and thats bad
decel fuel shutoff is a great thing, and the mpg gauge maxes because your using NO fuel, meaning you have an infinite mpg rating
it doesnt do it in nuetral because if it killed the fuel, you would have to restart the car using the key, where as when the ecu kiks out of decel fuel shutoff the engine is still spinning rombeing in gear, apply fuel and spark and the engine is once again running