The spawn will always yield ONE item from TWO of the four different deco stages. It may be the first and fourth at one spawn, the third and fourth at another, etc. The actual item that spawns, however, can be any deco item that has already appeared. So if something spawns at the third deco stage, it could be any item from the first, second or third stage deco pieces.
You can increase the items you get from a particular spawn by "farming it": if an item spawns at a particular point, you can let the spawn regress - stop killing and wait for a red skull to disappear, which is ten minutes provided you haven't added any further white skulls. Then start killing again and when the red skull is added back another item will spawn. Rinse and repeat. This is obviously best done at the fourth and final deco stage (when the second red skull is added at the third spawn level) so that you have a chance of getting any of the possible objects available.
You should also note that many items are very difficult to see until the static deco disappears at the end of the spawn. This includes the rock and lava tiles in the south, some parts of the pillory and some of the skull pieces in the vat. If seeing whether an item has spawned at a particular point, you often need to use cntrl-shift and count the number of items to be sure that a deco piece has not spawned and is lost among the static pieces.