Just unlocked special delivery and ODST 60G at the same time…
For…throwing two plasma grenades across the map at a robot…
When I got ODST it was delayed and I was in a hog. I know it was delayed because I remembered that right at the start of the KOTH match I was playing, I ran for the central hill and went up a grav lift, snapshotting one of the players and sticking another (before I died in mid air haha). Chances are you got it legit, it just took time for the achievement to pop 
No, I didn’t get it legit for sure. First game, and got 3 achievements in less than a minute for throwing 2 grenades lol
I got ODST by being shot by a rocket launcher at close range… I got another one for absolutely ridiculous grenade kill.
Even the Achievements are bugged…
I got ODST and special delivery for a double kill (1 from melee, the other from a plasma grenade that exploded on my face, killing another nearby) death from the grave.
…I guess the explosion caused me to travel as fast a person on a man cannon…?