Prone to confusion

I know I am picky about language, but I don't really understand why the word "prone", which means "lying on one's stomach/face down" is apparently taken by some people to mean something like "lying flat" (how else can one lie, anyway?!).

In "Life Sentences" by Alice Blanchard I read: "Now she stared at the ceiling from a prone position on her motel bed." Well either this character could turn her head completely around as in "The Exorcist", or else she must have been looking at the ceiling of the room below! Or, of course, she was actually lying "supine", i.e. on her back (face up).