The percentages on the Mastery page represent both your class's average completion of the learning pathway and each individual student's progress.
For example, 58% suggests that, as a whole, your class is 58% of the way done with mastering all concepts in the pathway.
Hover over your class average to see how far along the entire class is from mastering all topics in the pathway.
Additionally, Kyle Chameleon has an 80%, which suggests that, as an individual, Kyle is 80% of the way done with mastering all concepts in the pathway. Note that this data does include concepts that Kyle may have mastered by practicing independently on the site.
Hover an individual student's graph to see how far along he is toward mastering that entire pathway.