I don't know. It could be the result of a cut feature, and it was just easier to keep both items or they wanted to keep the garlic for the sake of Wario lore.

Update: There is a single distinction between the two. There is also a small Wario that you turn into if you are damaged. Small Wario turns into Normal Wario if he gets garlic, but Bull Wario if he gets the Bull Pot. I guess it's comparable to Mario's mushroom and fire flower, except if the mushroom also worked like a fire flower if you already have the effects of a mushroom.