I just opened both pictures in separate tabs, the browser says that both are 640x480.

But that's a good point nonetheless: That 640x480 picture is certainly upscaled somehow, otherwise it would be absolutely tiny on a retina display. That upscaling might certainly lead to very different results from both images.

However, when I download the images and open them in Preview, I get the same result. When I zoom in, I do see individual pixels. It looks like it just did nearest neighbor upscaling, so I'm not sure upscaling is at fault here.