So after scanning through this for a minute, I noticed that Mew, Celebi, and Shaymin have a 100% chance of holding a Lum Berry in a wild encounter in every game since they were introduced. Jirachi and Meloetta have a 100% chance of holding a Star Piece. Why is this? In most of these games they are unencounterable, but what is the purpose of this in the first place?
Through a bit of scrolling, I think I found the answer!
This is a list of Pokémon by the items they may be holding when caught from the wild from Generation III onward. Note that not all Pokémon listed can be caught legitimately in the wild, but their held item exists in the game's programming.
In other words, they're programmed that way, which implies this list was datamined and confirmed.
We don’t know why they programmed it this way.
Source
Hope this helps!