The advertised space for your HD is 320GB. The companies go by the "assumption" that there are 1000MB in a GB, which is wrong.
There are actually 1024MB in a GB. The math you did was how many ACTUAL MB are in your HD by multiplying 1024, the number of MB in a GB, by 290, the ACTUAL space on your HD.
Let's do the math that was giving in the example on the link I sent you but instead of 500GB, we'll use 320GB.
320.000.000.000 byte / 1024 = 312.500.500 Kbyte . .
312.500.500 Kbyte / 1024 = . . .
305.175 Mbyte . . . . .
305.175 Mbyte / 1024 = . . . . . .
8GB off, close enough
Those periods act the same as commas.