The way it works (as far as I have heard) is that when it's "fulfilled by Amazon", the vendors will ship Amazon their products.
Amazon won't really do much in the way of vetting them, and then just throws them all into the same bin (i believe they go by the SKU and not much else). When you order from any of the vendors that sell that exact item, they just pick one at random and ship it to you.
All it takes is one bad actor to ship a bunch of counterfeit stuff to amazon for fulfillment and then it's possible to get counterfeits from ANY seller that Amazon fulfills.