I was kind of wondering this myself, so this article definitely satisfied my curiosity. I don't have a Wii, & something like Manhunt 2 is definitely not my cup-o-tea, but there's definitely been some hub-bub around the game's release...

"In the AO-rated build of Manhunt 2, we could clearly see these over-the-top and horrific animations. In the M-rated version, Rockstar has added both an extreme blur effect and in most cases darkened the graphics so that it is nearly impossible to make any sense of what is going on."