I assumed they were UI elements. I haven't actually seen MS Paint in ten or more years, but I'm guessing it has many widgets that are square buttons with something inside, so the image data would appear as repeating patterns of identical length and have at least somewhat similar content (because everything is in a bordered box, and there's probably an "up" and "depressed" version of each, leading to nearly identical sounds being side-by-side in the data).

That's just a guess though. One would probably need to disassemble it and roam around a bit to be sure.