Invert node, why is it different form inverted image ?

Hi,

I have a BW image i use as a map for roughness. But it is inverted. So in my material node (cycles), I used an “invert” node to turn White to Black and vice versa. But the result is different from the one i get from inverting the image in the GIMP. Is it much brighter.
I also tried a “subtraction” node set as “1-value”, and that gives the same result as the “invert” node, so that’s a good start.

Why is it different ? Has it something to do with linear images and such ?

I dare to refresh that question. I don’t have a clue…