I had the same problem on my laptop which, like many phones, has a high density display. When a website hasn't explicitly provided support for high DPI displays, the browser is forced to scale up the source image (typically by 200%). To see the expected effect, you just need to lower the browser zoom level a couple of times to get the image back down to its native size. You'll know you've got it right when the A block suddenly appears lighter and matches the C block rather than B block (which will now look too dark).