With some products, that's entirely intentional. The goal is to convince you to keep thinking it was a good idea so you say you like your car in consumer satisfaction surveys that work as better sales drivers since people tend to do a degree of research on them, and to keep people from trying to return the product or make complaints.

But that's typically things like "cars", and they target you very precisely because they know exactly who you are, having just sold you a car.
The humidifier is that they see you bought a "home air treatment device - consumer grade - basic", which puts points in you being in the category of people who want to improve their home air quality. And you know what the most common purchase by that category of people is? Humidifiers!
As big and horrifying as the models are, they can't track everything and rely on putting people into consumer groups, and labeling products by relevance to groups and then tracking them that way.
So even though you're probably in the group because you just bought the thing that most people buy to put them in the group, it's still statistically the thing someone in the group is most likely to buy.

All that to say, it's a fucking sham and you get 95% of the results by matching ads to products instead of people, and it costs a fraction to do it that way.