Amazon doesn’t just fulfill Amazon.com orders. Anyone can send inventory to Amazon and use them for fulfillment on their own e-commerce platform. The distributors don’t know Amazon is going to be fulfilling orders from several of their retailers.

Even on Amazon, it’s not uncommon to find several new listings for an item fulfilled by Amazon from different sellers (including Amazon). That’s beneficial for Amazon because they don’t need to own all of the inventory and the sellers get a listing with good reputation to leverage if Amazon goes out of stock. In the perfect scenario everyone wins - Amazon makes money, the seller makes money, and the product is still available to the customer. You get all that without commingling, but with it, you also save physical storage volume.