User blog comment:Emptylord/Sandbox/Random Mutation/@comment-4948122-20130830085416/@comment-10958553-20130830164212

Actually thresh's passive works on an algorithm the same what TF's passive does. If you are below your "quota" for dropped souls, the chances for dropped souls is increased. The opposite is true for when you are above your quota. The random factor is important so that it will be harder to predict witch minions drop a soul, but ultimatly it is in no way random.