Mental depletion may bias the judgment of people. A situation where this bias manifests is the Hungry Judge Effect.
A similar situation could occur when the examiner tends to award less points in the end of a grading session. If the same student is always graded at the end, this may have a substantial effect on the points of students.
Shuffling could address parts of these bias as the students that are graded in the end are not the same for every exercise.