This is normal. When you purchased your new Tesla, you paid a retail price.
When you want to trade it back to Tesla, they can only offer you a wholesale price.
If you really want to know how much depreciation you have, then you need to sell your car, by yourself, in the retail market. This is the real depreciation.
Depreciation should also take into consideration any tax credits, rebates etc you gained on your initial pruchase.
When I privately sold my original Model X, I actually made money, due to all the tax credits, rebates and depreciation I was able to take.