I've read that these ads target you because by buying a dish washer your category went from "no idea" to "people who buy dish washers online". So while your chance of purchasing a dish washer right now might be low, as you just bought one, the calculated probability of you buying one has actually gone up.
Additionally, it alleviates the creepiness factor a bit ("they're so bad at tracking, they don't even realize I just bought one!", so you don't think about the perfect match with headphones you were just offered) and they might simply have missed the purchase.
They are just trying to help you out. They know there are "if you find it cheaper in the next 30 days, we'll refund the difference" policies out there. So those ads are actually much more helpful than you are giving credit. They can't help it you chose poorly and used a site that did not have that policy. /s
The point of the offer is that you don't have to do that. You just report that you found it, they verify, and then they will refund the difference in prices. Very few vendors do this, but it is a legit offer to help alleviate those post purchase regrets.
That's not the reason you keep seeing ads for products you just bought. Google was aware of your interest in the product yet was unable to track that you actually bought one. So they think you're still looking.
Additionally, it alleviates the creepiness factor a bit ("they're so bad at tracking, they don't even realize I just bought one!", so you don't think about the perfect match with headphones you were just offered) and they might simply have missed the purchase.