Are you sure about that? I vaguely recollect reading somewhere that the 3rd party vendors stocks were indeed shared as undifferentiated stock to optimize logistics, but that Amazon kept its own stocks separate over QA reasons.
For new products, that is how Amazon does it. Which will explain why many products will have inconsistent reviews about fakes. And you can ignore any review that says "Avoid seller X. He sent me a fake!" You have no way of knowing which seller's item you got.
Note that this is only for "Fulfilled by Amazon" orders. For non-FBA, the seller ships it to you directly.
And for used items, Amazon will ship you the exact one from the seller.
I should add a qualifier: What I say is true only for new products. For used products, they'll ship you the exact item the seller shipped to them.