For some individuals from socially advantaged backgrounds, college is a culturally expected outcome. For this group, college is less exclusively and intentionally linked to economic gain than it is for people in less advantaged groups, for whom a college education is a novelty that may well demand economic justification...individuals who are least likely to obtain a college education benefit most from college.
What follows is a statistical maze. I prefer a narrative that starts with the simplest way to characterize the data and then proceeds to introduce complications gradually as needed.
If I am reading their table 6 correctly, it suggests that majors in science, math, and engineering tend to come from the relatively advantaged groups, majors in education and business tend to come from relatively disadvantaged groups, and majors in social science and humanities tend to come from groups in between. I tend to think of majors in science, math, and engineering as having obtained skills, majors in education have obtained credentials, and majors in business, social science, and humanities as having obtained neither. That is a gross over-generalization, of course.