You must not just believe your eyes when trying to tell what's lighter/darker, just look at that illusion:
A and B squares background actually have the EXACT same shade of gray ! If you don't believe that, just put the picture in an image editor and look at the pixels' components: R=G=B=139 (except for some compression noise pixels).
The Sony screen background seems a little lighter and the darkest rectangle really darker on the picture (I checked with an editor) but there's some factors that can modify that, like uneven lighting (for the background) and software settings. I tried with the Customize View/Adjust View settings on the PRS-T1 and it can change both screen background and text/drawing shades of gray considerably.
