Thank you for the images, it helps me see what you're working with and trying to accomplish.
TL:DR - This may be a simple case of the wrong tool for the current job. Another workflow is likely what you'll want to use in this type of situation.
I'll explain a bit about Defringe and why this is the case.
This tool is built to help deal with fringe pixels that are created due to the anti-aliasing effects of selections used to cut and paste pixels. This anti-aliasing makes some of the edge pixels semi-transparent. Pixels within the selected area might become lighter, for example, when you are removing a dark object from a light background. This change would not be uniform across the entire edge of the selection but would depend on things like the amount of curve in that region. Similarly, the area outside the selection might be pulled in but with very low opacity.
Defringe looks at these edge areas and tries to even things out visually, mostly by either lightening or darkening these pixels but maybe with a little color correction as well.
To determine how much lightening, darkening, or color shifting may need to take place, it will look at pixels which are a little more inward from the edge. How far inward is what the Width setting actually checks. A setting of 1 or 2 pixels is optimal because if you stray too far from the edge you might integrate colors which are not near that border region, causing a different type of color contamination.
The Width setting does not tell Defringe how thick the edge area is. The decission on which pixels are going to be changed and by how much is based on things like how transparent/light/dark any given pixel is, how these qualities compare to its neighbors, and how they compare to the pixels at the Width location. The more anti-aliasing has occurred, the more the pixels will be changed.
That is why you see most of the changes in the curved part of your image and nothing really changing along the relatively straight sections in the upper right and the lower left. And if the pixels are inside the selection enough to not end up anti-aliased (and thus fully opaque, or nearly so), they would be ignored by Defringe even though the are visually different from the rest of the subject area. This is especially true of the thick light area in the upper right.
It's all a lot of very complicated but ultimately unsophisticated math. This is a very old feature, after all. It was made for a time when the selection tools were much more primitive. Taking advantage of better selections to remove these fringe areas would likely be the better solution. The controls in the Select and Mask workspace have options which are probably closer to what you are expecting from Defringe. Settings like Shift Edge and Radius are more likely to give you the results you want.
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