I can picture many different things matching this overall impression. Perhaps Alice is the child of two university professors and has vaguely drifted to near-perfect scores without really having to work. Alternately, she has slaved over her books desperate to please her parents and is at the end of her rope.
Similarly, Bob might be a self-educated polymath who has transcended his background, or a crammer who has carefully done everything exactly right to produce the highest score possible, but is almost entirely uneducated outside of the areas that produced good-looking admissions.
I do recall hearing that (at some point in history) the army looked into this and found that black soldiers did better then their white counterparts with an equal score entering. Further, this is presisly the result I would expect to see in the face of generic anti-black bias. For instance, if there was some force that depressed black SAT scores by 10 points and had no other effect, then a black student would be expected to perform as well as a white student who scored 10 points better on the SAT.
Since Bob came from an environment that did not value education, we might expect to see an improvement from the mere fact that he is now in an environment that does value education. In contrast, since Alice already came from such an environment, we would see no such improvement (from that cause).