Well, in the US people are desensitized from the overexposure of violence from the media, so it's no big deal. We just can't allow our children to be exposed to a naked breast, especially a cartoon video game breast .... (those are the worst kind btw), because seeing something like that could destroy their childhood, and we all know that a child's innocence is a precious, precious thing.
That was funny.
18+ for some gore in battles, that's just ridiculous when many violent movies are 15+
I think the difference come from the participation of the player in the violence. 18+ sound probable.
Oblivion was rated ESRB M and yet so was Gears of War, Grand Theft Auto, Leisure Suit Larry, and Condemned. Yet all of those games far exceeded Oblivion in the department of violence, sexual content, drug references, etc.
The rating doesn't indicate the quantity, just the presence. If there is one boob hidden in a cave 150 km under a mountain only reachable with a code, there's still sexuality content for the guy who rates it.