I'm trying to imagine a scenario where someone returns a single-purchase item and then purchases the same exact item as the result of seeing an ad for it. They'

I'm trying to imagine a scenario where someone returns a single-purchase item and then purchases the same exact item as the result of seeing an ad for it. They're already aware of the item and if they want another one after return they're probably just going to exchange it because something is wrong with it. It's not even a sale and I'm skeptical that seeing the ad has a strong influence on whether a customer asks for a refund or a replacement. ML is not some holy grail that always produces optimum algorithms. It depends heavily on inputs and on the assumptions, both explicit and implicit in the training data. My guess is Amazon does not have a "single-purchase item" model and it's not something the AI was trained to identify and so they aren't specifically excluded from groups of "similar-or-same" items. I think it is far more likely that Amazon is optimizing the cost of development against the marginal benefit of not showing ads for single-purchase items than that this is somehow driving more sales.