Car owners can decide the share per Km they would want to propose to the passenger for the ride. However, the cost share shown to the passenger is calculated for the actual distance of carpool for the passenger with the car owner. This means that you could see different carpool cost share for different matches depending on the amount you have quoted per Km and the length of ride overlap between the passenger’s and car owner’s ride.
Note that this is a suggested price and negotiation from the passenger side should be welcomed - as long as both are happy with the final price.
To change the fare, drivers can go to Settings -> Car Settings -> Price per km.