No, if you look closer you see the line surrounding the "Disk Usage 1337Mb", it's not like that anywhere else on the image.
I know and recognize what you're talking about, but I don't agree with your reasoning for its existence.
It's because mspaint is crappy at encoding JPEG's. Here's another example. I simply painted the background gray and made a red rectangle and saved it as a JPEG:
As you can see, the strange coloring around the line exists here too.