This sounds as though it might be a cultural difference. Where was the place in which you were staying?
In England, I provide a fitted bottom sheet though many people use a flat sheet that is tucked in. I then have a quilt on top, also called a duvet, to which I fit a cover that is removed between guests for washing. The quilt itself is washed, though less frequently.
It is not usual in Europe to use a flat top sheet under the duvet.
I have always wondered about the American "comforter" and thought that it was an additional thick cover that one might put on top of the normal bedding if extra warmth (or comfort) were necessary.
It's an interesting difference.
Debbie