Okay - imagine that the chest is a single 'line'. On one hand, you have a robot that is told 'put your stuff in the first slot you can'. On the other hand, you have a robot that is told 'take the first stack in line'. If it's the same 'stuff' in every slot, there's not going to be a problem (other than potentially filling up the slot with all of the same thing), but as soon as you have something else come in, it goes into the second slot. Unless your second robot is fast enough to make at least two 'pulls' for every time the first robot 'places', it's just going to keep taking the 'first stack in line'. If it can make that second 'pull', then it would take the second stack in line.