When I disobey the don't-zoom directive ("These examples will only work if your browser doesn’t do any rescaling on the images below") I see what your screenshot has. Try pressing Cmd+0 (command zero) to reset your browser zoom to Actual Size and then the pattern should match C instead of B.
EDIT: Sorry, the above applies to my non-retina external display that I have hooked up to my retina macbook. When I view the test images on my internal retina display, I do see the issue you describe (pattern matches B). If I press cmd+- (command minus) a few times until I'm at a 50% zoom level, the issue is resolved and the pattern matches C! Makes some sense actually, since showing a normal dpi photo at 50% on a retina makes for a 1:1 pixel mapping :) Showing an image at 100% on a retina makes for a 1:2 pixel mapping (each pixel from the image ends up being 2x2=4 physical pixels), which disobeys the don't-rescale directive.
EDIT: Sorry, the above applies to my non-retina external display that I have hooked up to my retina macbook. When I view the test images on my internal retina display, I do see the issue you describe (pattern matches B). If I press cmd+- (command minus) a few times until I'm at a 50% zoom level, the issue is resolved and the pattern matches C! Makes some sense actually, since showing a normal dpi photo at 50% on a retina makes for a 1:1 pixel mapping :) Showing an image at 100% on a retina makes for a 1:2 pixel mapping (each pixel from the image ends up being 2x2=4 physical pixels), which disobeys the don't-rescale directive.