I'm going to guess UPS keeps warehousing separate per vendor.
https://classic.qz.com/perfect-company-2/1172282/this-compan...
You have a product with a UPC. You have a picker with a UPC and a scanner. If the scanner scans the wrong UPC, the picker grabbed the wrong thing.
And yet I get sent the wrong item from Amazon multiple times per year. Ballpark of 1 in 20 orders. And not like I got the wrong order. More like I ordered spicy taco seasoning and receive regular taco seasoning. So a mistake that makes sense if you were matching on titles but why not match on UPCs?
Amazon will only then use your items for your sales and not for anyone elses. Possible longer shipping times, etc. But I s always been possible to ensure correct items on the seller side with FBA.
Most people like to use the same SKU to get more exposure on amazon.com though.