Trebuchet pierce armor, packed vs unpacked.

Can someone explain how the armor works on a trebuchet when it is packed compared to unpacked.

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I noticed that when I clicked on my unpacked trebuchet it says it has 150 pierce armor, but it was getting killed in one shot against a group of elite conquistadors. When the trebuchet is packed it says it only has 8 pierce armor.

After some testing, the elite conquistador is doing 10 = 18 (damage) - 8 (armor) damage to a packed treb, but it is doing 6 damage to an unpacked treb. If the treb has 150 PA unpacked, shouldn't the conq only do 1 damage instead of 6?