i have over 13000 kills already on leaderboard and no i'm not hacking
If you have it, try getting a few more kills. Sometimes its awarded that way. Especially if the patch is something new.
Or it could just be unable to track your progress.
Or you could try to contact Support via Twitter or in Live Chat to try to sort this out, but be ready to spend 1-2 hours on this, as it takes a while to get into live chat.
i think it might be 10000 kills with guns only, not counting sentry guns