Because the block sizes are not multiples of 8, it's not possible to descramble them without transcoding, so the resulting file is a PNG despite the source being JPEG.
Be aware that (at time of writing) if you were to try extracting the final rendered images from the page elements at the source website (yanmaga.jp), the images you get will have double the compression artefacts because the site transcodes the temporary canvases to JPEG before displaying them for some reason.
