Some manufacturers measure the "firebox" including the area above the baffle. This is disingenuous IMO.Your measurement is probably more accurate for the actual usable capacity.
Before giving up on the stove. Take a good straight-edge and put it across the front top face of the stove where the door gasket makes contact. See if that area is flat and true. Then do the same thing for the door edges. If the door is warped, insist that the dealer replace it under warranty. If they suspect overfiring, make them prove it.
I am not giving up yet. I e mailed the company nothing yet so I will call them next week. Called my body who is a welder see if he can do something. I took a picture hope it works it is hard because of the angle.