Some tiles can be placed on top of another tile.
With some tiles, the rules state that the tile below is discarded, with other tiles, they state that the tile below is just left where it is.
What difference does it make?
With regard to the base game and its included expansion "Saga of Edda" it makes no difference. Therefore In the implementation at http://www.yucata.de/en/GameInfo/Voluspa the tiles that would be underneath are always directly moved to the discard pile.