This is a fundamental economic concept - the more of something you have, the less value you derive from each incremental unit.
So, based on personal preferences, once you reach a certain level of income you will start to substitute away from labor (income) towards leisure/free time (non-market activities such as hobbies, cooking, parenting, gaming, chilling).
The study data shows this pretty clearly (if I'm reading it correctly) - in Study 2b (wealthy people polled at a science museum) the median household income was $100-149k and the "% Time Oriented" was 69%. In Study 4 (online panel members who get free internet in exchange for survey participation) the median household income was $75-85k and the "% Time Oriented" was 46%.
The implied result that upper middle class folk are able to spend more time on leisure and are therefore happier on average than regular middle class folk doesn't seem too groundbreaking.