Consider the question of shared memory. I think it does just that - first uses the dedicated 256Mb of graphics memory and only then shares mainboard memory upto the level stated. If you don't need more than 256Mb then it won't necessarily use any mainboard memory and if you do need more you may be very pleased it does so as to preserve performance.
If you bought the computer locally you could try returning it. Otherwise you have the Distance Selling regulations that allow you to return it but you are running out of time for that possibility.