Android content transition problem

gistfile1.txt

//Setup how the content elements should animate
TransitionSet transitionSet = new TransitionSet();

transitionSet.addTransition(
  new Slide(Gravity.TOP).addTarget(R.id.header_container)
                        //Why do i have to exclude this?
                        .excludeTarget(R.id.content_container, true));

transitionSet.addTransition(
  new Slide(Gravity.BOTTOM).addTarget(R.id.content_container)
                            //Why do i have to exclude this?
                            .excludeTarget(R.id.header_container, true));

transitionSet.setInterpolator(AnimationUtil.interpolator());

detailsFragment.setEnterTransition(transitionSet);