Yes, it appears the formula works differently in Attila than in Warhammer (or rather, works differently if armour value is greater than incoming damage). Going by Walrus Jones' post, I think this is how the damage can be calculated (see
his thread):
So, in this case I guess the average damage should be as follows:
Snaga damage on average: (3 dmg/2)*(3 dmg/10 armour)=0.45 dmg per hit
Uruk Hai damage on average: (3 dmg/2)*(3 dmg/3 armour)=1.5 dmg per hit
So, in short, it does appear that the *other formula* only applies in situtation where the amount of soakable (non-ap) damage is greater than the armour value.
Edit:
Comparing the effect of armour values of Gondor Sword Militia and Sword Infantry:
Militia received damage in melee on average: (3 dmg/2)*(3 dmg/8 armour)= 0.56 dmg per hit
Infantry received damage in melee on average: (3 dmg/2)*(3 dmg/12 armour)=0.375 dmg per hit
And against archery:
Militia: (5 dmg/2)*(5 dmg/8 armour)=1.56 dmg per hit
Infantry: (5 dmg/2)*(5 dmg/12 armour)=1.04 dmg per hit
I guess the main thing to take home from this is that the "one hit kills" you were aiming for (if I understood you correctly) are not actually happening with the current system. Indeed, it seems that you really need to use a 1 HP system if you wish to include that effect, or just decide to scrap the "1-hit kill concept" if you wish to use a multi-hp system.
Edit2:
Then again, Walrus Jones also states the following in his thread:
However, if you actually calculate the damage in this case using his formula you get this result:
Average dmg=(20 dmg/2)*(20 dmg/40 armour)=5
Which would imply that the attack should do 5 damage on average (in other words, always 1-hit kill a single hp unit). So, perhaps these average damage calculations are not that accurate.
Edit3:
Actually, the formula is accurate, it's just that it doesn't really apply to 1 hp situations. With a 1 HP system, the only relevant calculation is this, as it determines whether all the damage will be absorbed by the armour:
Average damage on hit=AP+((Damage/2)
(Damage/Armor)).
So, yeah, the average damage calculations in this post should represent what is actually happening in-game.