I already expect, for example, that some people have 127 and some have 128 for the mid gray, and truly, neither of these is .5. The goal is to neutralize that value and to do that you subtract whatever it is. Here we go again with math - sorry can't show it visually.

and yet, you just did in that statment. that made sense. no math, just a plain explanation. I now understand why thinking  .5 - as 127/128 grey - is an issue*.*