I decided to look up the two games on esrb.org. For Halo: Reach, they mention "Characters are occasionally impaled on glowing swords", "Gun fights are highlighted by realistic gunfire sound effects, explosions, and screams of pain", "players are able to shoot dead enemies, causing more blood to splash out", etc.
Now let's look at Uncharted 2, which got Teen (it got the same content descriptors as Halo in addition to Suggestive Themes and Language). "sneak up on the enemies to perform "stealth kills" (e.g., neck-snapping, choke-holds)", "brief splashes of blood are emitted from characters; more shots—and thus, more blood—are required to kill a growling ice beast"... and then they go on to mention innuendos and language. There's much less of a focus on talking about violence.
So there you go. You get Mature by allowing impalements and shooting dead enemies. Uncharted 1 had to take out shooting fish else it would have gotten slapped with a Mature rating. So shooting animals gets you Mature too.
In other words, yeah, the ratings make no sense. Some games push the M rating more than others. Heavy Rain allows you to possibly cut off a character's finger.
Manhunt 2 was the only game I saw get an AO rating for violence. You'd think if Manhunt 1 couldn't get AO then no amount of violence would. I understand it got it for having less of a reason for the character to do the killings and that the killings may have been even more violent. They had to take out a few things and introduce a blurry tinted filter to get it passed as M. But man, I don't think you really could get much worse than the first game. How is a chainsaw to the face in the first game only M material?
EDIT: Then I find out they did release Manhunt 2 for PC download-only, AO rating.