reducing protection, multiple attacks/round

[jeffr]

I attacked a few Bane Lords that had a protection of ~25 and luck pendants with about 80 longbowmen with flaming arrows. As the Bane Lords drew closer, many arrows would hit a Bane Lord in a combat round. Yet, they took almost no damage.

I thought that each attack after the first one reduces the protection of the unit by one. So, lets say 40 arrows hit the Bane Lord square. Of these 30 hit the Bane Lord (just guessing here on the 30 out of 40, don't know the formula). 15 get through the luck shield. Shouldn't 15 flame arrows do some damage to a ~25 protection unit?

Or am I wrong about missiles reducing the protection by 1 on each successive hit in a combat round? Is this only with melee attacks?

Do the missiles (or a melee attack, for that matter) have to cause damage before reducing the protection, or do they just have to hit?