Seems to a fundamental US / UK disagreement over this, but part of it comes down to language.

In the US, it seems normal practice to have a sheet over your mattress (of course), then another sheet you sleep beneath, and then a "blanket" (by which I presume they mean a duvet / quilt) on top of that. Having the top sheet means you don't come into contact with the "blanket" and therefore don't have to wash it as often, if at all. Seems many Americans find the idea of not having sheets to be disgusting, even though you can of course wash duvet covers as easily as bed sheets and pillow cases etc.

In short, that top layer sheet is pointless, aside from being an alternative to a duvet on a very hot night.