Well, your ability to earn a chest does not affect your grade at the end.
Many factors go into your grade. How good was your kill participation? Damage done to champions? How about objectives you took or assisted in taking down? How many wards did you place? How was your cs? How long did the game last?
These are just some of the things that we know for a fact affect your grade. Any one of them can offset another (most commonly, supports get s ranks for good kill participation, wards, and structure assists. As Zyra, damage done would be helpful for getting a higher grade as well). Also, deaths dramatically decrease your chances of getting an s. Also, shorter game times increase the grade you get, so that may be a factor here.