Why not. All grading is arbitrary, when a professor marks an essay or any other piece of writing and gives it a random percentage the exact same thing occurs.
Assignments usually have specific criteria that form the basis for adding/subtracting points.
What are the scoring guidelines for something as personal and subjective as whether or not you had fun with a game? Do you approach it on its own terms, or some universal good game checklist? Maybe a guess at how you think the average gamer will feel? It's dramatically more slippery than grading an essay or a test, with infinitely more room for interpretation.