Math, Physics, and Chemistry are easy to objectively grade, so I'd accept that they tend to have higher grade quality (a notional metric of the correlation of g

Math, Physics, and Chemistry are easy to objectively grade, so I'd accept that they tend to have higher grade quality (a notional metric of the correlation of grades to ability) than e.g. literature, drama, sociology, pottery, etc, where the grade quality correlates to the ability of the grader. Physics classes don't prove a blanket statement regarding grade quality over all subjects, any more than Yao Ming proves that Chinese people tend to be seven feet tall, or the King of Sweden proves that Swedes tend to be kings.