I'm not sure, but it could be something to do with how much time you spent in multiplayer in the previous games. I didn't get to 100 in GoW2, but I did get this achievement.
The description says "Your skill and experience means you’re ready for Standard Versus multiplayer." so it must also be unlocked once you play 3 for long enough as well, as there is a "beginnner" style lobby in Multiplayer to start off in, but I skipped
over that because I had the achivement already.