If multiple sellers sell the "same" item and use amazon as warehouse, they can be binned together and you get one at random from any one of the sellers no matter which one you actually bought from.
Yup, they call it "commingling". So even if you make sure it says "sold by and ships from Amazon", and Amazon itself only buys from legitimate vendors, you could still wind up with a counterfeit item introduced by a third party seller.