According to the system logs you killed the required for the achievement NPC 5 times yesterday: 10:13am; 10:19am; 10:37am; 15:06pm and 15:13pm.
Then at 15:13pm Achievement was completed with 30/30.
For the moment you started the thread you killed the required NPC (
http://archeagedatabase.net/us/npc/9792/) only 3 times so had 28/30 progress only by then. Two other times (10:25am and 10:31am) you killed a different NPC (
http://archeagedatabase.net/us/npc/12191/), that is not the one required for the achievement.