Gareth--

I suspect it's just an informal clarification of an admitedly ambiguous American usage.

THE Sheriff is the elected head of law enforcement in a county. But we hardly ever bother to say "deputy sheriff" when speaking of his many underlings. We might say "a deputy did this," or "a deputy said that," but I think just as often, we would say "the sheriff" even if we meant a deputy. Therefore in ordinary usage "sheriff" can me any member of the dept. You could make it clear that you meant the boss by saying "the high Sheriff," though you're right, he wouldn't be formally known as that.

CC