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.