Can someone help me understand the logic behind "Preferred Plus" versus "Preferred"?

For example:
777 - 23AB is Preferred Plus, but 23HJ is Preferred. Why is left different than right?

777 - 24AB is Preferred, but 33AB is Preferred Plus. Why is it better to be in the back?

737-800 - 15A is Preferred, but 15F is Preferred Plus. Again, why is left different than right?