I would not be so sure several items are actually stored on the same spot, but I would rather say the first item is stored normally, and the other ones are just put down on top of it.
That most probably results from a situation where your Hearthlings could not find any available spot for these payloads and just dropped them on location when stuck, in order t ofree themselves for other tasks.
To me, that results from race conditions when attributing tasks, and this problem would naturally be solved if those did not exist in the first place.
I made an attempt at describing this superseeding problem in another topic