Basis Residual value calculation Leasing resetters

Hello dear ones! Now there is also trouble with us! We leased a Citroen C2 VTS 1.6 HDi FAP 3 years ago. We started on the 0% financing at Citroen then, because the offer sounded good. It is of course a residual value leasing. At that time it was assumed as a basis for calculation that the car goes back after 3 years and then it has 60,000 km on the speedometer. So far so good. The guaranteed redemption value is now, according to the contract, 7,728.00 €. Today I have with said m dealer called and let him know that the flicker has only 36.000 km on the speedometer and we would hand it off including winter tyres on aluminium wheels, sports radiator grill and iPod port (of course, everything was not available ex factory, but was installed afterwards, but can be removed again). He then gave me the following information: If I would return the car in a perfect condition, then the vehicle value would be at 6,100 € at present. n! According to Schwacke, the car has a dealer’s purchase value of 7,300€. Now my question to you: What is the basis of the dealer’s calculation? I mean, he can come and say that the car is in principle not worth anything more. The car has cost new almost 18.00€. I am stunned that it is supposed to be worth only 1/3 of the new price. And that after only 3 years. What experience values do you have about this? I thank you already in advance for your answer en!!!