The collision hull is not the same as the bounding box. There are several different collision methods. Models have normally an ellipsoidal hull or a polygonal hull. The bounding box is around that shape and can be retrieved by vec_for_min/max. The min_x/max_x coordinates are external settings for restricting the collision range. You can see them by pressing F11 twice.

Look here: http://manual.3dgamestudio.net/collision.htm