It´s just a survey, but one thing we can conclude by the data: those who earn more, "think" they are more happy. And those who earn less, "think" they are less happy.
The point is, what is happiness ? By these numbers, happiness comes from money and by my personal experience (could be wrong), happiness don´t have anything with money. Maybe joy, satisfaction and fun have, but not true happiness. So the article could be entitled as: The Self-Employed Are The Most Satisfied ... and not the happiest. Maybe the most happiest are priests, nuns and people who dedicate their lives to others. Guess the survey was ok, but the journalist behind the article was not happy in the way that he interpreted the research.