Indeed, the former system (FNI - Fichier National des Immatriculations; Registrations National File) made number and thus plates to change when the car was changing department. But it gives an indication of the first registration of the car in a department, which can be useful for identification of the model year when it was the very first registration.
And it allow to see some mistakes

But with the 2009+ SIV (Système d'immatriculation des véhicules - Vehicles Registration System), the id is given for the entire "life" of the car : you don't have to change the number when changing department (but still have to declare it to prefecture ).