I am not sure how it works in Blender, but it should be similar. The spawn point (crosshaird) is the 0, 0, 0 origin in model space. That is, in Max when I set the Z = 0 position as "ground level", I would position the wheels to rest on that plane. Be sure to check world/local coordinates, to be sure you are taking the absolute measurement for Z and not the relative one. However, this may possibly be a FDM issue. Check the Z values for the gear BOGEYs in your XML, as the landing gear points may be below the wheels, and therefore the aircraft is resting on that, with the wheels above the ground.
Best regards,
Uriah