Odometer values for the vehicle are stored in the CCN control unit (which is the "brain" of the instrument cluster).
I can't recall where the mileage is stored outside of the CCN, but the engine control module knows some odometer values; eg the odometer reading when the last DPF regeneration took place.
Odometer info is probably in other control units (there are 22-24 electronic control units in a US/Canadian NCV3 Sprinter - not all US/Canadian NCV3s have the hands free module for onboard cellular service; not all Sprinters have the multi-disc CD changer control module).
It doesn't sound like the cluster was replaced properly.
During the procedure, the scan tool is connected to the vehicle, the technician follows the prompts on the scan tool software interface (a computer screen).
1) connect the scan tool
2) read the info on the cabin compartment node (CCN) electronic control unit (the instrument cluster & its "brain")
3) leave the tool connected
4) remove the instrument cluster
5) install the instrument cluster
6) click "next" to populate the new instrument cluster with the old cluster's info