"Relaxed atomic operations can be reordered in both directions, with respect to either ordinary memory operations or other relaxed atomic operations. But the requirement that updates must be observed in modification order disallows this if the two operations may apply to the same atomic object. (The same restriction applies to the one-way reordering of acquire/release atomic operations.)"

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n233...

There is a lot more explanation here that should cast some light on the situation: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n248...