For most Items, (let's use a Relic Crown as an example,) go to their Bulbapedia page, and you will see two interesting things. They can only be found in the Abyssal Ruins of BWB2W2, but they have a bag description for every game after that.
This is mostly prevalent in Key Items, (probably because most normal items are reoccurring,) and am wondering why Game Freak would bother to do any of this in the first place, as it seems to be a waste of time, (unless they're basically copying large amounts of data inbetween games.)