It has nothing todo with accommodation strain, it's an "exposure" problem. If you are looking at a bright screen in an otherwise dark room, your pupils will adapt to the average flux hitting your retina. But if most of your field of view is dark, with a bright screen in the middle, this will cause an uncomfortably high flux from that part of your field. With a more even lighting so that the brightness of the screen is close to the average illumination, this doesn't happen.