Because this exam gives your employer better understanding of your ability, and Cambridge exams don't have an expiration date so as soon as you get your result, it will stay with you forever.
If you do badly on a test, you may be tempted to believe that you've received some fundamental information about yourself from the professor, information that says you are a failure in some significant way.
And so there is going to be a discrepancy, naturally, between minorities and between whites if they go to better schools. And they will not do as well on exams because they haven't had as much help.