> I've noticed the drywall in that area of our house, too, is really unusual, or at least I'm unfamiliar with it. It's like regular drywall but with an added layer of relatively strong plaster over the surface.

Someone likely had the wall "floated" to either fix some wonkiness in the drywall without tearing it down, covering up something like old wallpaper, etc etc. It's a fairly common thing to do to drywall.