That's only in the stats of Destructive Power, Speed, Range, Durability, and Precision. Developmental Potential is calculated differently, an A rank there means there is still plenty of room for improvement, whereas an E means that all potential has been met.

Well stands are manifestations of a person's personality and spirit. Kira is a strategical genius and up to the point where he comes into contact with the gang he had never really been challenged and never had to actually apply himself in order to get what he wanted. Had the whole ordeal been longer and more dragged out there's no way to really know what levels of OP his stand would have reached.