In my school, when the exam sheets were returned to the students after scoring, it was a common practice among students to compare the answer sheets with other class-mates and more than often Y student would realize that although he answered similar to X, he still got lesser marks for a particular answer. Y would then run to the teacher with his and X's answer sheets and try to argue about the validity of score. Surprisingly the frustrated teacher who might have had his/her own reasons to score the students in a certain way would send X back and deduct another 1 mark from Y's total score just to discourage students from comparing the score with each other. Idea was to encourage students to argue on something on their own merit (e.g. by bringing the text-book or a more valid proof to claim the marks) rather than comparing with fellow students.
After my BS, just for fun I taught 5th, 6th and 9th graders in my aunt's school for 6 months. Also during my PhD, I got chance to teach undergrad students in the USA. My experience also says that sometime it becomes easier to ignore a similar mistake by one student in comparison to other when
1) you realize that overall knowledge/understanding of X is better than Y or
2) you see a tremendous improvement in X compared to his performance in previous exam and you as a teacher would want to encourage X to continue performing better.
Thanks for reading!