That is exactly how ADDSORTED works. It breaks ties randomly. The same can be said for items were not even answered. Since they will all have an empty value they are sorted randomly.
So for your constant sum question if I had 8 items and only 4 of them were answered like so:
The items could be sorted in ascending order either 6,3,7,2,1,4,5,8 or 3,6,2,7,8,5,4,1.
Hope this helps