Grades seem arbitrary, subjective and open to abuse by bad teachers and schools. Percentile seem to be objective, indisputably defines positions etc.
Looking at grade boundary statistics where A* is awarded from about 4 pc students in some subjects to about 50pc in others. Even after removing outliers where only few students take the test, range is between 4 pc to 20 pc; that is quite a large range.
Even if there is an argument that exact percentile is unfair, atleast it could be defined in range.