[jefftk]

I don't think it's a mystery. Ad targeting usually doesn't know which people have just bought a vacuum, just that someone has been looking at vacuums recently. People who have been looking at vacuums are far more likely than the typical person to buy one, so it's not surprising that it would be worth showing them an ad reminding them that they can buy one from you. Especially with higher margin products like mattresses or cars.

But how do we explain how there are also advertisers like Amazon who do know that you just bought a vacuum from them and still show you more ads for it? Since Amazon is in a position to run principled A/B tests on whether showing these ads leads to sales that otherwise wouldn't have happened, and they are the kind of organization I'd expect to get this right, this part I am willing to accept without external evidence. It's probably that the likelihood of additional purchases of the same item, for yourself or others, is high enough, combined with that the cost of advertising to you is low enough.