I took a plasma pistol, which seems to have hybrid damage type ( half fire, half energy ) and shoot someone with metal armor. Armor used it's full DT for fire and energy damage types. I think it should be using only half of it's DT. Here is a simple example that will hopefully explain my point of view:
We have two weapons, a longsword with 10 slashing damage and a morning star with 5 blunt and 5 piercing damage. Both weapons are equally strong.
If they attack unarmored target they will both deal 10 damage: longsword - 10 slashing, morning star - 5 blunt and 5 piercing.
What happens if a target has an armor with equal protection against all three damage types? Let's see.
Target has an armor with DT 4 vs slashing, blunt and piercing damage types. It gets attacked by longsword:
10 - 4 = 6 slashing damage.
Now it's attacked by morning star:
5 - 4 = 1 blunt damage, 5 - 4 = 1 piercing damage. Together it's 2 damage.
Something is not right here. Armor has equal protection vs slashing, piercing and blunt damage types so it should protect equally vs long sword and morning star. But it isn't.
Now, let's divide DT by number of damage types of a weapon.
Longsword damage = 10 - (4 / 1) = 6 slashing.
Morning star damage: 5 - (4 / 2) = 3 blunt, 5 - (4/2) = 3 piercing. 6 damage total.
This time it works as expected
If we replace DT with % damage reduction then armor with equal protection against slashing/blunt/piercing will reduce damage of longsword and morning star by the same amount. No division is required if we use % damage reduction, but in case of absolute reduction, it is.