By default, Amazon has a setting turned on in a Seller's account to use the manufacturer barcode for their items. This is also known as Stickerless Commingling
To disable Stickerless, Commingled Inventory if you would like:
Seller Central->Settings->Fulfillment by Amazon
Scroll down to FBA Product Barcode Preference
Click the Edit button to the far right
Select Amazon barcode and click Update
Note: Any changes you make to this setting does not take effect until you create your next batch.
For more information on this topic, feel free to check out Amazon's Help Page.