The dithering might actually indicate that this is an 8-bit PNG (as opposed to 32-bit), or it's somehow a lossy-lossless PNG, as the lower filesize would suggest. Zooming into the image, it becomes clear that the colour-depth is degraded. Also, although it is indeed higher resolution than the parent, it doesn't appear to be any sharper (there's isn't any additional high-frequency information), although I don't have any objective measurements for this.