Board Thread:Questions and Answers/@comment-80.130.41.154-20160606002034/@comment-25501246-20160606141454

It's based on averages, like every statistic. Midoriya barely used his Quirk at the start, so he just gets an 1.

Same logic for a lot of other characters like Tokoyami, who restrains to use his Quirk when it's at its strongest.