I think you're referring to the "bell curve" in grading, which some teachers use and others do not.
That's quite different from what "learning curve" means.
Let us know if you have further questions.
Grading of students is an individual thing. Although the "bell curve" (normal distribution) is used by many teachers, where they make the cut off points for particular grades is an individual affair.
When I first start a particular course, I use the normal distribution. However, if over the years, I notice a consistency in student scoring to my type of tests, I make specific cutoff points for various grades.
Here is the grading scale I used.
Your final grade will be determined by total points on an overall curve of all sections of this course. Excluding the grade insurance (mentioned below), the highest possible score is 360 points (i.e., four 50-point exams, a 100-point final, and four 15-point papers). The approximate grading curve based on total points earned for the course is indicated below.
295+ = A
255+ = B
215+ = C
145+ = Pass
Within 10 points below each of the above grade designations will be considered "on the line." The higher grade may be awarded at the instructor's discretion. The instructor's discretion will be related to unexcused absences, class participation and other related factors. If an exam is missed and is averaged, the instructor's discretionary area will no longer apply.
However, my courses were at a college level. I do not know what your level is. Even so, if a student wanted to know why they get a particular grade, I am will to try to explain it to them.
It seems that you are not sure whether your teacher has a specific grading criterion for the grade change or she just cannot justify the change and does not want to admit it.
If you feel the change is unjustified — and this is extremely important to you — you could take the matter to the teacher's supervisor. However, this is likely to result in ill feelings from the teacher that might bias her grading even more. You have to decide whether this important enough to take the risk.
I hope this helps a little more. Thanks for asking.