It's not floating; it's sitting on top of that ruined wall. I've tried to jump up there to get it (to no avail).

That is the top of the wall, you dork!

I have to agree as well, as I see the bottom of the chest completely. if it was sitting on the wall you wouldn't see that. The right hand corner of the top of the wall comes into contact with the edge of the chest but you can see the clouds

Okay, it's possible that it might be floating but it's still on top of that wall.