It's caused by the quick and dirty image scaling algorithm, it doesn't take enough samples per pixel. We used to see this sort of stuff in computer games before

It's caused by the quick and dirty image scaling algorithm, it doesn't take enough samples per pixel. We used to see this sort of stuff in computer games before mipmaps were invented, which are smaller versions of the image pre-scaled using a fancy algorithm and then used in place of the larger image when it's smaller on the screen.