How to use the fragment stack?

Question

Java

I have 3 fragments called one after another, each one is added to the stack, when I close 3 fragments, I call popBackStack and 2 fragments open for me and more components from 3 fragments are visible on it. Why?
Here is the menu

@SuppressWarnings("StatementWithEmptyBody")
    @Override
    public boolean onNavigationItemSelected(MenuItem item) {
        // Handle navigation view item clicks here.
        FragmentTransaction ftrans = getFragmentManager().beginTransaction();
        int id = item.getItemId();

        if (id == R.id.nav_teory) {
            ftrans.replace(R.id.content_main, fteory, "ftheory");
            setTitle("Теория");
            ftrans.addToBackStack("ftheory");

        } else if (id == R.id.nav_test) {
            ftrans.replace(R.id.content_main, ftest);
            setTitle("Тест");
            ftrans.addToBackStack("ftest");

        } else if (id == R.id.nav_exit) {
            Intent intent = new Intent();
            setResult(RESULT_OK);
            finish();
        } ftrans.commit();

        DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
        drawer.closeDrawer(GravityCompat.START);
        return true;
    }

Here in the 2nd menu item I call the 2nd fragment

fuser = new FragmentUser();
                        FragmentTransaction ftrans = getFragmentManager().beginTransaction();
                        ftrans.replace(R.id.content_main, fuser);
                        Bundle bundle = new Bundle();
                        bundle.putInt("Points", Points);
                        fuser.setArguments(bundle);
                        ftrans.addToBackStack("fuser");
                        ftrans.commit();

Here are 3
getActivity().getFragmentManager().popBackStack();