From what I understand, the overall points differential only comes into play if teams play each other more than once in pool play (I'm not sure if which tournaments this happens? Perhaps paul would know?).
ie. if teams play each other twice, and split the results one win each, and happen to score exactly the same points as each other during those two games, meaning their goal average would be 1, then overall point differential comes into play.
But in the Olympics, when teams only play each other once in pool play, it doesn't come into play, except to classify teams 9-12.
Confused yet?