Did you open the app and download the digital key prior to the car being given to you? That is what officially starts the booking, and if you did that during the delivery then it would explain why it added extra miles on.
Alternatively, it may be that the entire delivery was charged to your account and there were already some miles on the car to start with. There will be some when the car is delivered from Renault to Onto. More when Onto are using the car to install their equipment, and then further miles added taking the car from Onto to the delivery depots around the country. It sounds perfectly possible to me that this would explain the problem.
EDIT: @Tomahawx beat me to it!