Multiple factors at play here. 1) Rubrics are often defined, but the application of the rubric is by a human. Application will shift as the grader gets a sense

Multiple factors at play here. 1) Rubrics are often defined, but the application of the rubric is by a human. Application will shift as the grader gets a sense of the classes understanding. 2) As you get fatigued while grading, you'll make mistakes, and be less tolerant of others. Especially if you're an overworked adjunct or graduate student. 3) There are probably a lot more last names early in the alphabet so weighting is important. My policy on this when I was a grad student was to publish the rubric, and ask all students to check their grades too.