As you know, there is 4 ways to get items from monstres while in fight; Kill, Overkill, Steal and Ex steal.
The theory is that each ennemy have one overkill item, a set of "normal" items and a set of "rare" items. When you steal, you take one item from the "normal" item pool. When you Ex steal, you take one item in the "rare" item pool. The Kill drop will come from any of the two item pool. The only way to get the Overkill item is to overkill the monster. The overkill items are often "unique" items (they are found on one monster only).
After some experimentation, we can suppose that if you steal all the items, you will not be able to have this item as a Kill drop. However, if one item is in the "normal" or "rare" item AND is the overkill item, then you can get it twice.
This is one example of one early monster test (easy to kill). The only item that was not obtained is the Fragment of life in Ex steal and cure grass from kill (however, it's in the table so I will give credits and say that it's a kill drop).
This example can almost prove that the theory is good. However, there is some monsters where I was not able to confirm it, one of them is the Imp, the drop are the following
- Overkill - Horn of Devilkin
- Steal - Horn of Devilkin
- Kill - Silver Ore
- Ex steal - Silver Ore
Then, according to the theory, we should get this.
- Common -> Horn of Devilkin
- Rare -> Silver Ore
- Overkill -> Horn of Devilkin
However I never got any Horn in kill (but it should).
The, we can guess that the drop rate is different for each item. As you know, you don't always get items when you kill, steal or ex steal, so there is a success rate for each drop. On top of that, I think that there is a drop rate for each item. It might be possible that the kill drop rate for the Horn is really low, so you almost always get the "rare" item as a drop, in other words, "rare" items can be quite common, it did not mean that it's a rare drop.
We would like to modify the monster pages to adjust the drop items, but we need to know if you think that this item drop theory is good. Feel free to test this theory or propose something else.