Yes, but then it was discovered that there are nipbles in the game that you have to use mods to find and of course while torture, murder, and violent bloody death are perfectly fine for teenagers to be exposed to we can't allow them to see human body parts.
it wasn't just nipbles. the nipbles only caused the ESRB to re-review the game, whereupon they found such delightful things as Lucien Lachance hanging naked and skinned from a ceiling, and a room full of corpses and blood and a decaying head, and decided "hey, maybe 13-year-olds shouldn't be seeing this".
Skyrim will be M guaranteed. the only reason it's not yet is because it's not in a finished enough state for them to worry about shipping off a once-over to the ESRB.