Grades are an issue fraught with hot spots, disappointments, anger, frustration, and misunderstanding. Suppose you worked hard on a research paper and think yours is an "A" paper, but when you get the paper back, your grade is a "C." Most students are angry or disappointed, but few do anything about it.
When you receive a grade that you don't understand or if your instructor has made a mistake when grading a test or paper (yes, it happens), don't just sulk and think that the instructor is out to get you. Don't tell yourself that there's nothing you can do about your grade, because there are things you can and should do. While an instructor may be uncomfortable when asked to justify a particular grade on a project, as a student, you have every right to ask for an explanation.
For example, on a paper, you may ask your instructor why you received the grade you did, especially if the comments don't clarify why the grade was given. Don't go into the meeting with a defensive attitude or with a chip on your shoulder. Instead, go in honestly seeking to understand your grade so that you can improve the next time. Ask your instructor specifically what you did well and what you did poorly. Ask him how you might do better on the next paper. Your instructor will appreciate your efforts to understand your performance and seek ways to improve that skill.
In some cases, the instructor may reconsider the grade if you explain your thinking behind your work. If you explain why you included certain material or why you organized the paper a certain way, your instructor may reconsider. Even if your grade isn't changed, at least you have a better sense of what the instructor looks for in that type of assignment.