You are talking about measuring happiness as if it was an objective phenomenon. What if the interviewers of this study had no clear idea of what happiness really is in first place? What if there isn't even such a thing as happiness as a normative concept? It's not like people don't need to read tens of self-help books, and learn meditation and research spirituality to have a mere and ephemeral glimpse of what happiness is really about...

Asking people how good they feel about life doesn't help since people are delusional and have the wrong assumption that being successful and wealthy is being happy. Following the same standard you would find that people are happy during bull markets and depressed during bear markets. This is a very broken definition of happiness. Given that the study covers only USA residents, a country which is known for being an extreme example of materialism and consumerism based culture, it's likely that the bulk of the subjects have a very strong bias toward thinking that happiness relates to fulfillment of the "keep up with the Joneses" social urge.

This study seems more like a exercise in proving the trivial tautology that people who have enough money cease feeling that they don't have enough, and any extra money has little effect in the very same way that beyond some amount of food people stop feeling hungry and any extra food just makes them feel bloated. That's just mistaking the feeling of being financially secure for happiness.