What I understood is, that your kills will count for your achievements, but these ones won't count in the leaderboards.
That means that, In the strange scenario that your achievement unlocks exactly when you kill 10.000 people, If you check the leaderboard and count all your kills, you may be able to count a total of 9.900 kills, but get the Seriously achievement.
At least, that's what I understood, but.... I'm Spanish... so maybe isn't that xDD
P.D: Playing GoW right now, so If you find me... invite/join :P