I think this is how the system works but it's probably wrong: You kill all 6 barrows brothers and say 20 other things. 2 of the monsters you killed (brother or other things) are randomly selected and their drop is placed in the chest along with all other drops of that item so the server selects karil and a skeleton, karil drops a coif but nothing else could have so only one goes in the chest, the skeleton drops 8 chaos runes but you've also killed many monsters which dropped them so you get 432. Ofcourse this doesn't explain that sometimes you get chaos, deaths and mind runes as well as gp.