The number of kilometers driven is calculated by subtracting the mileage indicated at the check-in from the mileage indicated at the check-out. Since this is an automatic process, please make sure both mileages are correct before confirming. See this article for more information on how the process of extra kilometers driven during a rental work.
Do you have a Keyless car? Read more about mileage registration here: Keyless: How does it work with mileage registration?