I don't  understand what you're seeing that's wrong.

(I'll number the bricks/nodes 1 - 4 starting at the left for ease of writing.)

 

4 is the source image, right?

3 removes 128 of the green element of the source image.

2a multiplies the green-subtracted result by 1 (the value provided by 2b).

1 adds in 128 of the green element.

Result? The same as the source image since all you've done is remove 128-green, multiply by 1, then add 128-green.

 

Did I miss something obvious there?

 

EDIT: Ah, it's the in-between delta colour you're pointing out is different. I was concentrating on the result and not the working-out that led there, sorry!

But even then, there's nothing out of the ordinary, since the Poser node set-up is not subtracting pure 128-green. The colour tile in the colour-subtract node is a combination of all RGB elements, so the result of subtracting it from your source image is going to be different compared to subtracting just 128-green.