Emphasis is always placed on more recent levels of performance, rather than focusing on earlier attempts where students were still learning the skill or concept.
How One Weird Finding Changed My Perspective on Grades
Arthur Chiaravalli

Makes so much more sense than an average. Doing this fosters the mindset that you enter a class to LEARN. You may not know much when you enter, but you will learn a lot, and then you have new knowledge and understanding at the end. Why should the beginning states of not-knowing influence your grade at the end. Should you not be evaluated, at the end, on your new, current knowledge and understandings — how you are a reader/mathematician/scientist NOW.