I am a little confused why a sequentially consistent memory system needs to maintain all of those orderings. If X and Y are different locations, why should it matter which order they are read from or written to? Writing to location X doesn’t seem like it would change what would be read from location Y, regardless of the ordering.

I guess my understanding is that there can only be one memory operation happening at a single time, and each operation must complete before the next one starts, so it just happens that these four memory operation orderings are respected by definition.