All the exam papers are hand corrected and manually totaled. During the grading process, the graders tend to give marks which total to an even number. For example, say the total up until the previous question is 23. Then if the next question is for 6 marks, the grader might assign 5 marks for that answer to get the total so far to 28. Mental arithmetic of even numbers is easier and this saves the grader from going over two passes to obtain the total at the end. All of these several hundred thousand papers are manually graded between the end of exams in April and the declaration of results in June.
The graders only work with a roll number and school number so they have no information about the identity of the student when grading. This has nothing to do with the caste system as some people here are suggesting. The identity of the student is matched with the roll number *after* the papers are graded.