XD 40k balance.
It happens sometimes, lets say.
I am killing 2 banshees.
http://i.imgur.com/oT2WQEN.png
I killed one but since my configuration to loot is loot last it wont loot the 1st banshee till I kill all the monster in my screen, is this case the 2nd banshee.
So the body of the 1st banshee is around there, but I am running and I killed the banshee right here:
http://i.imgur.com/6Z1m56y.png
As you said, sometimes because of the distance is large the bot wont loot the first banshee, however when the bot go again to that place, eventually will loot all the corpses that the bot doesnt looted the first time.
Hope that I explained myself.