I marked thousands of papers as a TA in grad school. I was in the humanities, so there was quite a bit of room for subjectivity in the marking. After a while, I found that the average mark tended to go down toward the end of a 4-hour marking session. It could have been caused by decision fatigue. Or it could have been irrational annoyance, i.e. first time: "Hmm, he made a minor mistake here." 70th time: "Why on earth does everyone keep making such an elementary mistake?" Anyway, I compensated by consciously giving out higher marks toward the end.

Anyway, if I'm ever accused of a crime, I'll tell my lawyer to do whatever he can to schedule a session shortly after lunchtime!