Most things improve when covered with melted cheese.
I have all ways referred to them as scalloped myself, I like both depending on what with.
Have a nice big dish of leftover au gratin in the fridge
Mashed with meatloaf any day.
Au gratin and scalloped aren't the same thing. Au gratin has cheese...scalloped is just a milk or cream-based sauce. (there can be some other differences...but they're usually minor.)