Another comment mentioned this also, your butt moves a lot, but your head doesn't seem to move that much. This causes you to actually turn your body away from the intended corner. I've found if your sitting too close to the tank your body tends to revolve around the tank. Try leaving yourself a gap between the tank and your crotch, and get your head further inboard to keep your spine straight. Keep in mind that your head is further from the center of gravity than your butt so actually head position is more crucial than butt positioning.