I think a part of the discrepancy between grading in different subjects is that how you can find a grade is different. So if my final exam in a math class is 10 problems, and they are worth ten points each even if you sub divide the problems to allow for partial credit, it is a lot clearer what level of work is an A, a B and so on. I think it is easier to leave the room thinking "I nailed it" or "oh no..." and to be correct and when you get the test back it will be clear where you messed up. For an English class my final exams were essays, and since my ability to write an essay was no longer being tested (that is it was not, 10 points for a clear thesis, 5 points each for supporting pieces of evidence... and so on) what is being tested is my analysis. What was my thesis, and was it convincingly supported, did I miss some other plot point which disputes my argument or even something major which seems to support it but which I ignored and so on. I think it is a lot harder to leave the room knowing how well you did and when you get your grade back a lot harder to understand it sometimes, or to know how to study harder for the next one.
I think that in math you get a percent grade, and then that is converted to an A,B,C,etc where as in an English class you are given a "average, above average, excellent" and then that is converted to an A,B,C etc. Though I have had essays come back with percents on them, I always wondered what made this essay a 91 and not a 90 or a 90, what small part was worth that one point?