Editing the image at the pixel level would also produce similar results. But, is that really plausible?
Why would anyone want to make it less like the original by monkeying around with settings about which they are probably clueless?
I lean towards Occam's Razor here.
Also, this is "Show and Tell" time. I've shown you mine, now you show me yours. ;-)
Editing the picture at the pixel level would be just grounds for an accusation of forgery, since any part of the picture that it is edited in such fashion would no longer be derived from the scanned document.
Why would anyone want to make it less like the original by monkeying around with settings about which they are probably clueless?
Although most programs that save JPG files just have one 'quality' setting, the quality setting in fact sets two parameters. Different programs may set the parameters somewhat differently in response to a "quality" slider control. Certainly it would not be unreasonable for someone who is producing an image for web display to crank down the quality setting to reduce file size.
I lean towards Occam's Razor here.
Careful. In the case of the Rathergate memos, there was no possible explanation for the way the documents were formatted that was consistent with them being genuine. It wasn't merely unlikely that genuine documents would have been forwarded that way--barring the existence of time travel it would have been absolutely impossible. One would not only have to believe that the author of the memos (1) had access to extremely fancy and expensive equipment; (2) would have used it, rather than a pen or pencil, in producing a 'memo to file'; and (3) would have written such memos to file anyway. One would also have to believe that either (1) Microsoft designed Word based upon common settings and behaviors of a typesetting machine that nobody's ever seen, (2) Microsoft designed Word based upon the typesetting conventions of some obscure memos whose existence would not be generally known until years later, or (3) the author of the memos decided to use many typographical conventions that would have made no sense in 1972, and just by coincidence happened to stumble upon the details and behaviors of Microsoft Word's default settings. Or else one would have to believe that the documents are the only observable evidence of time travel.
The Obama COLB has nowhere near that level of mendacity. It exhibits no features that had clearly not been invented until after it was allegedly printed (in 2007). It would seem unlikely that the Hawaiian records office would have made a change to the formatting (including the border) and then reverted that change, but absent testimony by the people responsible for such things, stating that no such changes have occurred, I could not regard such a thing as impossible. Further, while I don't know of any JPEG programs that would yield the fringing seen on the Kos COLB without deliberate independent adjustment of Chroma and Luma quality settings, I would not be surprised if such a program exists. While it's impossible to imagine someone like the 'author' of the Rathergate memos having access to equipment that would be obscure and horrendously expensive, it's not hard at all to imagine that some Kos or Obama staffers would include some rather obscure software in their repertoire. I know I use some software few people have ever heard of, and I doubt I'm the only one.
The Obama COLB smells fishy to me, but unlike the Rathergate memos it could possibly be genuine. A number of improbable events would have had to occur for it to appear as it does, but the odds aren't so low as to banish the possibility into fantasyland.