"Games such as Harvester, Soldier of Fortune and Manhunt have shown gruesome violence, yet only got the M rating, which is supposed to be the entertainment software equivalent of the R rating. Many critics have claimed that these games deserve the AO rating[3] and were given the M for commercial reasons...
...Critics have claimed that the ESRB will only rate games AO if they have sexual content in them, regardless of how much violence is present. Twenty three products have been given the AO rating. One was given it for unsimulated online gambling. The rest were given it for sexual content and/or nudity."
This is from the same Wiki and probably explains why they are hiiting Manhunt 2 so hard - trying to prove a point.