My computer does that to. SATA is hot swappable, on many boards, just like usb is. There is nothing wrong.
Hard drives are advertised as 1gb= 1,000,000,000 bytes when its actually 1,024,000,000 bytes or something along that line.
Plus formating takes up some space too.
Its fine. My 2x250gb in raid 0 shows 465 gb total after formate.