This seems like yet another collision detection problem. However, the bomb has a fixed range which can be obstructed by obstacles, at your case, the tombstone. This tombstone from my experience "absorbs" some of the range, making some zombies slightly hiding "behind" it take less damage. The bots are in a fixed position with minimal obstructions so they are usually taken out with ease though zombie positions are highly variable with lag allow them to sometimes live. It is likely a lag problem with the game being confused of where the zombies are or a bad game gimmick with the range of the bomb. Sorry but I can not confirm this though it seems to be the likely problem.
