Remember that our car shuts down 3 of the 4 cylinders when it hits that 100+ mpg readout and although this is run by the third cam profile I'm sure it does take just a bit of time for the car to note this through the gauges. I have also had this happen to me - middle of gliding, while accelerating slowly (
mph readout climbing) and IMA bars showing charge, descending a hill with constant throttle, etc. I dont' think it is an issue and cure it simply by applying or lifting the throttle slightly. I just chalk it up as one computer telling another what it is doing but not telling the third, etc. No issue I bet.