Amazon credits you when someone buys your item, but the item Amazon ships the buyer might not be yours, but an equal equivalent.
And, before you ask: Yes, this results in situations where 100% legitimate sellers on Amazon can sometimes ship (via Amazon/FBA) their buyers counterfeit goods. Shitty situation to be in as a seller.
Sellers can opt-out of commingling, but many don't because a) they don't understand the implications, as Amazon doesn't really make it clear; and b) because it requires them to individually add a separate sticker/identifier to each unit of their product, adding significantly to preparation and packaging costs.