Myth #1: The practice exam had nothing to do with the real exam. 

This is the most common complaint I see about exam grades. The irony of this complaint is that my practice exams are literally real exams that I now use as practice. For any particular course, I will have at least a few versions of the same exam. Here is how I make a different version of the same exam, typically: I will make a copy of the exam on my computer, name it something like "Exam Version 2", and slightly or moderately reword the questions. Over time, I designate one as a practice exam and make this available to students. Therefore, if you feel that the practice exam was nothing like the real exam and vice versa, you are quite mistaken.

Then why would you feel that the practice exam didn't seem similar to the real exam? This is likely because you have fallen victim to the well-known Dunning-Kruger effect, which is the tendency of people with little knowledge in a field to greatly overestimate their knowledge. If you are taking an introductory course (or even if you have taken several courses) on a subject, your knowledge of the subject is not sufficient to (a) fairly assess your understanding and (b) to assess the similarity of one exam compared to another. 

Myth #2: I deserve a higher grade because I need to (insert reason here).

The second most common complaint I get about grades is typically of the above form. While I appreciate your argument that you need a higher grade than the one you have earned, this does not entitle you to get a higher grade. Consider the following argument. 

Let's assume that I grade all students using the same rules and criteria (which, by the way, I do). Therefore, if you come to me with this request and I grant it, I would have to grant it for all students, right? And don't you think that all students need or want a higher grade? Thus, I would end up giving everyone an A+ and there would be no point in exams or assessments.

But, you retort, you really need a higher grade. Other students might say they do, but you really do. How am I to evaluate this? By what criteria am I to judge who needs higher grades and who doesn't? There is no such criteria. Neither you, nor I, nor anyone could come up with such a system.

Even if it were ethical to give grades based upon what students need or want (it isn't--see the next myth), it would be impossible to accomplish this in a way that would be even remotely fair. Thus, what you're really asking is for me to offer you special treatment. I have never, and would never, offer any student special treatment.

Myth #3: Professors make up grades for students.

I often get the strong impression that students believe the above myth, although rarely do students say it explicitly. Perhaps students think that professors conjure up grades from an "infinity stone", or from some other poorly-conceived and overly-cliched fictional object typically featured in contemporary Hollywood movies. 

Actually, I don't know what grades students are going to receive until the end of the semester. I have an Excel sheet that I populate with student grades throughout the semester, and calculate the final grades using exactly the criteria I mention on the first day of class. I never decide what grade a student "should" get.

But what about assignment and exam grades? I always have a rubric, and I grade all students based upon the same rubric, to the best of my ability. In the case of online exams, the grading is done by an impartial machine; I am therefore removed from the process entirely.

It would be very unethical to make up grades for students, or grade students based upon disparate criteria. This is one reason why you cannot receive a higher grade than the one you want, need, or believe that you deserve; and, this should be reason enough not to ask.

But aren't grades still subjective? This brings us to Myth #4.

Myth #4: Grades are subjective and don't really indicate a student's performance in the class.

This is a corollary of Myth#3. This myth, although fundamentally false, contains a smidgen of truth. There is usually some level of subjectivity to grading. A little experience, however, goes a long way to exposing this myth. 

No system created by human beings is, or can be, perfect. There is not time here to expatiate on the follies of utopian thinking (read some Dostoyevsky, who often criticizes utopian thinking), but it is sufficient to say that utopianism seeks to create a fictitious world in which unsolvable problems no longer exist.

Let's take this argument to its logical conclusion: there is a modicum of subjectivity in grading, and therefore grades are really just a reflection of the whims of one person--i.e., the professor. Therefore, a grade is not reflective of any underlying performance or knowledge. Therefore, it's not really legitimate. Therefore, we should change how grades are assigned. 

How, exactly, do you plan on accomplishing this? Should we let students determine their own grades? What about professors assign grades, students complain, and then professors assign grades based upon who "needs" a higher grade or who complains most vociferously? What about we have an outside committee determine grades, presumably composed of people who know little to nothing about how the course is taught or the subject matter? There is no way to completely remove subjectivity from grading; all of the above suggestions, and I would argue all alternative suggestions, would likely introduce far more subjectivity than currently exists. 

In my experience as a student and professor, grades are generally a good indication of student performance and knowledge in a course. Looking back on the grades I received that I thought were unfairly low, I now realize that almost all of them were deserved.