Why would the first dimension necessarily represent the quality of the student? I don't see why that should be the case. Note- I'm not challenging you- I don't

Why would the first dimension necessarily represent the quality of the student? I don't see why that should be the case. Note- I'm not challenging you- I don't see why it shouldn't be the case- I just don't follow your reasoning (but I'm quite interested in the claim). Is it because the largest determinant in which answer is correct is the quality of the student?