a) Some students have no idea what they are getting into when they register for a class. (This explains some of the clueless students who can't do any of the assignments.)
b) Some students who register for a first course in their probable major subject register conservatively, especially if they can't prove completion of equivalent courses. (This explains some of the students who easily do more than what is assigned.)
c) Some students have some good preparation (e.g., math) but not all the best possible preparation (e.g., previous programming experience). (This explains some of the students who seem to find the course just right for them.)
Further along in a computer science major sequence, students who neither get it nor like it will move on to some other major. Then the ability spread in any one class may diminish somewhat.
b) Some students who register for a first course in their probable major subject register conservatively, especially if they can't prove completion of equivalent courses. (This explains some of the students who easily do more than what is assigned.)
c) Some students have some good preparation (e.g., math) but not all the best possible preparation (e.g., previous programming experience). (This explains some of the students who seem to find the course just right for them.)
Further along in a computer science major sequence, students who neither get it nor like it will move on to some other major. Then the ability spread in any one class may diminish somewhat.