Android : getChiledAt returns two items?

In onListItemClick method I use

lv.getChildAt(position-lv.getFirstVisiblePosition()).setBackgroundColor(getResources().getColor(android.R.color.darker_gray));

But background of two elements is changing ex. clicking first element (index 0) #0 and #8 change, clicking second item (index 1) #1 and #9 change, clicking 9th item (index 8) #8 and #1 change. What is going on?