here is a short explination of this
when you buy a new hard drive there is nothing on it, think of this as say, an empty room, just 4 walls and a door to get in. the hard drive has to be formatted (creating a file system), which would be like putting shelves in your empty room. The shelves in your empty room take up some space in the room, the same way with the file system on a hard drive, the "shelves" for the data take up some of the room on the hard drive, so thats why it's reading only 195 gb or whatever, because of the file system taking up room on the drive
like on my primary hard drive, I have a partition for my os and progs, which is 50 gb, it only shows at 48.8 gb, and I have the rest as another partition which is 150 gb, but it shows as 141 gb. and my 250gb drive is showing at 233gb, which leads me to believe that the bigger the drive the more space you lose. you can't have your cake and eat it too, windows doesn't work like that.....lol