Erratic average fuel consumption display

Hi have a 2013 Megane CC 1.5 dci, on my old 2011 coupe model I have an instant fuel economy and average fuel economy display.On this model it just has average fuel economy on the display. But the odd thing is it acts like an instant display going up and quickly as I accelerate or back off. On my old model it would take time during the drive to alter the average fuel value.

So do anyone know if 1 is should have both displays instant and average or 2 does any one else have an average display that moves up and down very quickly

Would be appreciate any info or advise as i am puzzled

Erratic average fuel consumption display