It's a somewhat uneven comparison, as the Megane held an higher speed, but it's interesting to note that the Auris mileage curve is still climbing after nearly 100 km.
Megane mileage curve instead has peaked and is going down.
The peak is related to a stop at a service station.
But why it suffers from that?
Maybe the sudden stop drops temperature?
Let's add coolant temperature to the same graph:
That doesn't seem to be the case: there's a drop in temperature, but it affects the Auris, not the Megane.
By the way we can observe that the diesel engine has a lower operating temperature than the HSD and the Auris reaches working temperature after nearly 20 km.