And basically the resolution of the zbuffer that is used to track hidden surfaces by the video card has some effect on this all as well, and when you add more objects to the model the range of values that are used for the zbuffer will change, and so that's why you could see some differences in hidden surface display after you add an object.

Some of these kinds of things are just baked in to how 3D accelerated video cards are set up to work.

- Michael