You're right, and I guess I've been breaking that rule for a while. What's the purpose there? The double-underbar and underbar-capital rules seem to be allowing for non-conflicting introduction of keywords. Is the single-underbar rule to protect standard library headers or something?
Reserved identifiers in general are not only for future use of the standard, but also for internal use of implementations and for extensions. The single-under bar rule is to give the programmer some part of the _... namespace which is otherwise reserved for the implementation. I.e. for use in macros, variables, or struct members. The intention is for use in hidden names in some API.