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I can't tell you if it's true for Reach since I don't kill myself for Kills, but in Halo 3 you had to compensate for lag after you died.
Get a "Kill from the Grave" too soon after you die and it doesn't properly register that you killed them after you died.
My advice would be to try get the kills as long after you died as you can. Don't stick someone, die, blow two opponents up a split second later and expect to get it.
For example, imagine two dudes are camping behind a door on Sword Base and you come up from behind them. You throw a grenade and immediately get hit by the Sword guy that is always camping somewhere. The grenade flies for a second and a half and detonates
about a second and a quarter/half after that.
That's a good two and a half+ seconds after you died, so if I'm right and that's what's causing your problem, it would fix it. Experiment a bit with it and see whether you can get a proper Kill from the Grave medal on just one person with varying tactics
(to see how long you have to make the delay to get the medal). Once you've done that you can use your new, reliable plan and tactic on two people for the Achievement.
Again: I could be wrong but I do not think so. Good luck .