Problem with relative grading system? My college (IIT) follows a relative grading system and each professor was given freedom in choosing how to grade the students.

As far as I know, Some professors give 5 students (less than 8 percent of the batch) the grade of S and others give A grade to 6-7 people in the class. I am looking for to apply for Phd, where I see colleges asking for a certain percentage in marks, like X percent in a taught module. 

It is almost impossible for me to know how much I scored in a particular subject as we were just given relative grading. How should this issue can be resolved?
I think grad schools are used to dealing with different grading schemes.  This is one reason, why GREs exist also.  But they can probably infer that top 10-20% is an A.  Bottom 50% is bad, etc.  Maybe even implicitly compensating for differences in how competitive the undergrad is.

Maybe grad schools aren't perfect but they seem to even be able to (in the US) evaluate students from all over the globe to try to pick the better ones.  Although even the US is very different from school to school.