From my understanding and conversations with George (EGW)..9x23 is slightly tapered, same length, smaller caserim than 38 super..the case is slightly bigger than super and slightly smaller than 38 super at the case mouth.
My 9x23 guns will chamber and shoot..9x23, 9SC, 38 SC, 38 super ( if I adjust the extractor ) and 9x21 and 9x19.
My 38 super will shoot 9x23 ( if the brass is new, once it's fired it will not chamber) 38 SC and 38 super.
don't think you are missing anything..they given tolerances..it is possible for them to all interchange..whether that is a good thing or not..that's up to you..