Because the lines are more high than the rocks and generate the shadow :) .

As i said i Know the chest not is in perspective, and is a fast try to see how looks this perspective. The final perspective will be like the other dungeons.

I don't understand the shadows of the white lines, they make the walls looks hollow.

Actually I don't understand the white lines, except they're nice ;)

 love the red tinting on the edges of the rocks and used throughout the other tiles. The coins are ace and remind me of playing with legos :D Overall, very good palette and pixeling :D

The white borders look a bit too bright on it, and the rock texture under the shadow just looks dull.  Also, instead of perfectly square tiles, you should make the bottom and top rows of the walls to be rectangular so you can add a bit of perspective.

 It's well pixeled, but the perspective on the chests doesn't seem to make any sense to the eye -- it makes me a bit dizzy.