J´ai trouvé ça sur le forum officiel (mais comme je suis pas trés fortiche en anglais):
From what I understand from the thread linked thusfar the following issues affect the "Seriously" achievement:
1)Remote suggests: All kills count (towards seriously) as long as you stay until the final "locust/cog won the match" screen. This is when the kills you gained during all the rounds are recorded to your hard drive. Even if everyone else quit except you and the host then the kills you gained will still count towards seriously (although probably not towards the leaderboard).
a) Other people quitting during a game does NOT affect your seriously kill count as long as you wait until that final screen.
b)If host quits or you quit none of the kills will count towards seriously or the leaderboards.
2)Clutcher suggests: If you get past a certain number of rounds, you may have kills that register on the Leaderboards even if the host quits. However, this number may not go towards the "seriously" achievement (i´m guessing as this number is not recorded to the hard drive as per the above). This would explain why you see more kills on the Leaderboards than should be necessary for the achievement but you still haven´t gotten it.