Okay so someone took a stamped ballot envelope, scanned it, deleted the fill on the triangle, the rectangle, and each of those letters, and repeated this process across multiple envelopes ... for what purpose? That's a lot of work to construct something that looks wrong. Amateur forgers are always lazy. They only change the part of a document they think matters, and they leave the rest of it alone. They're not going to play with random graphical elements. Professional forgers wouldn't make this mistake once, and certainly wouldn't do it repeatedly.
Because that's a red triangle, and the algorithm was able to detect the black ink of the stamp on the red background--but not perfectly, you can still see where it deleted some of the stamped letters. For obvious reasons, it could not detect the black ink of the stamp on the background of the black triangle.