Image sampler not reading grayscale images correctly

When using an image in Grayscale mode the image sampler reads the colors as if the image had a much lower bit depth.

In this example I use two apparently identical images. Both are JPG. The left one is saved from Photoshop in Grayscale mode (Image -> Mode -> Grayscale), the right one is saved in RGB mode.

This is reproducible in some computers but not in others.