"Presumably what is standardized though is the quality of work designated by letter grades. So in that respect, regardless of the percentage grade attached, an A is an A and a B is a B. "
I don’t know if you want to presume that, there are variations even within a high school wrt grading consistency. Teachers are different, they don’t all follow the same standard. A common example is if exams and quizzes are 90% of your grade with homework 10%, which means you can get a B on the exams (85%) and get an A in course because by and large the teacher will give 8-10 on the h/w (they check only for completeness).
Now yes, the tests may be designed where most kids get in the 70s and 80s but they know those will be curved up because of the homework or problem sets or labs or essays. Another teacher in the same high school may not do that.