A high school student is trying to decide whether to go to college or begin working after graduation. The student wants to know how the college degree will change their earnings, so they compare the earnings of one of their cousins, Fred, who has a college degree, with the earnings of their other cousin, Bob, who does not have a college degree. Fred earns $20,000 more per year than Bob. If you were advising this student on their decision, why might you caution them against using the $20,000 figure as a metric of how much more their earnings will increase if they go to college?