> I've always wondered what the steelman is for curve grading.

assuming that by "steelman" here you mean "the justification", i believe the point is that a curved grade shows how you compare to others. the idea is that "getting 40% of the answers right" is meaningless if you don't know how hard the test is, so you'd rather have a grade that says "top 5% of the class".

this what i see as the justification, at least. not an endorsement of the idea