The MPAA, now more than ever, are picky about movie content. Someone spitting blood, or one F word, or hell, smoking a cigarette can make the difference between G/PG/PG-13/R/NC-17 nowadays.
I personally think they release 'Unrated' versions as opposed to calling them 'R' rated so underage youths can purchase them without too much hassle.
9 times out 10, the 'Unrated' versions aren't worth buying. Most times the studio will throw in a three second breast shot, or an extra f-bomb or two and call it 'stuff you couldn't see in theaters', which, a lot of times, is bull, because the 'unrated' stuff is just extended scenes and added dialogue.