Apparently the whole of the tablet is not flat, if you try to put the tablet face down(display face down), you will notice a arch or a soft curve on each side, if you turn the display on, you can even see the light shine out, you will also notice the back is not totally flat either, put the tablet on a flat surface and you will see it. I have check this out with many S2, so if you using a TPU case, you will get this problem, many case makers won't deal with this problem and will just consider it to be flat.
Let me know if you get the same as I have mentioned.