>>46581410>>46583001>>46583064Items appearing in balls is a holdover from Capsule Monsters, which essentially revolved around monsters and objects being stored inside Dragon Ball capsules with a different look. Well, that, and what
>>46583462 said - they're just meant to represent any item.
Poké Balls have never been shown to hold anything but Pokémon in any official Pokémon media, so the chests mentioned in
>>46582802 make more sense.