Exactly, let me explain (this not directed to you Mackilroy):
Harddrive capacity is given in base10 (where 1GB is 1 000 000 000 bytes, and thus a 750GB drive is 750 000 000 000 bytes).
Software / OSes most often calculate 1GB as 1 073 741 824bytes and thus 750GB = 805306368000 bytes
Ah 750GB (750 000 000 000 bytes) drive is then according to the OS 750 000 000 000 / 1 073 741 824 = 698,49GB
So there's always a bit of a mismatch between the specified space and the space you "see" in the OS. Even though it is the same exact amount of bytes.