zhuge,

The reason general's bodyguards last so long is that they have both very good armour and 2 hit points each. So it takes awhile for "kills" to accumulate before they begin falling. You have to kill each man twice before he falls. So if you hit a random 10% per volley, you can see that it will be a few volleys before you have much probability of 2 hits on the same guy. Same thing happens with chariots and elephants vs. ranged units. That is why you don't see javelins or slingers or archers killing elephants after prolonged attack. Melee on the other hand tends to score multiple hits on a single character in rapid succession (because of localized killing, much of the unit might not even be engaged.)