Mass media entertainment companies are all about the money. They like to take a movie that you are well familiar with, perhaps even one that you saw at the movie theatre, and somehow put a new spin on it, that will make you want to see it again. While there are plenty of different spins, such as "director's cut", "extended version", and so forth, the easiest and quickest way to get that almighty dollar is the "unrated" version. For the uninitiated, unrated, sounds like it's on the edge, that whatever you saw in the theaters or wish to see is going to now be something way and well beyond those limits. Unrated--sweet!
The truth of the matter is far less envelope pushing. The creators of these films are large media organizations that have no interest in pushing the boundaries of the public's taste outside of a respected norm, and these movie creators are well aware that they need the imprint of the Motion Picture Association of America (MPAA) and their "voluntary" rating system in order to get nationwide distribution of their particular movie. When a specific movie is presented to the MPAA there will be a give and take as to what will or won't be allowed within the editing of the movie to achieve that final rating certification grade. When certain big-budget firms are on the edge of a particular rating, they will use an experienced and connected go-between to work out the creative differences between what is desired by the studio and what is acceptable to the MPAA, in which smooth talking, political skills, and a certain charm will pay dividends for both parties.
Therefore, when cuts or modifications are made to a movie in order to achieve a certain rating, directors and editors of said movie are well aware as to what scenes were deleted, modified, or truncated and that version of the movie still exists as an alternate unrated and uncertified copy. Consequently, it thereby becomes relatively straightforward to present in the future for the DVD set, an unrated version of the same movie with the previous scenes that have been excised or compromised in some way, becoming part and parcel of the DVD that is released to the public.
Of course, that is just one version of how an unrated DVD comes into being, a far more cynical version, is a director or editor of a particular movie, just adding in some additional curse words, or nudity, or more graphic violence, that previously weren't part of the original intention or version of the movie being released, but for purposes of marketing an unrated version are tagged onto the DVD in order to capture those unrated DVD sales.
In actuality, unrated DVDs are for the most part, the same as the movie, with a modest amount of more scandalous material added to it, in order to lure in unsuspecting buyers that they are receiving something that is substantially more in substance than what was shown at the movie theaters and to whet these viewers addled-brain appetites for more excess, when in actuality it's pretty much to do about nothing.