1. Y is worse than X. If you give both of them a math task ("how should I launch this rocket so that it lands safely"), then X is more likely to get it right. The reason why is irrelevant; results are relevant.
2. In contrast to what people like you usually think, performance and context are two different variables. If you measure performance objectively, that gives you a chance to influence context and see which interventions make sense! Maybe giving poor kids' families money doesn't help as much as sending the kids to boarding schools, or enrolling them in extracurricular activities. How could you know? By measuring it! Bottom line, objective measurements are THE BEST way of achieving positive social change (assuming it's your goal... it is mine).
Note that this isn't really exactly what grades measure much of the time. In an ideal world they measure mastery-- but they often measure a whole lot of classroom conduct, who's able to spend extra time on assignments at home, details of information presentation, etc.
My students who I'd judge most likely to get a critical problem right aren't always at the top of the gradebook. Sometimes they're near the bottom. And this is despite me weighing demonstrated capability and mastery very highly.