According to this Fangraphs article (from one year ago), Granite is the sixth best prospect in the game, projected to amass 10WAR over the first six seasons of his big-league career. Presumably a lot of that value is centered around his defense.
You probably want to change "Granite is" to "Granite was."
From the article: "It incorporates age, offensive performance, defensive performance, and other characteristics from the past two seasons."
I am going to stick my neck out and guess that incorporating 2018 stats into this methodology has dropped Zack's projection a little.