An assertion without any justification, not very useful. I will bite, why?
-Ice Armor/ Chilled Defense
-Crippling Blows (stacks)
-Armor buff/Frontal Defense buff
??
Everything below has been confirmed by Luke in the past.
There is confusion between "Damage reduction" and "Damage resistance". They do not refer to the same thing and do not work the same way.
"Damage resistance" is a reduction multiplier applied at the (afaik) end of the damage formula. E.g. Say you have a 30% damage resistance. Then it's dmg dealt by enemy = 0.7 * (end value of damage formula). Or something really close to this.
"Damage reduction" is a reduction that works by subtracting the preset damage of the targeted enemy at the end of the damage formula. However, keep in mind that this preset value does NOT scale with the difficulty. How this works is a bit ambiguous, so I'll explain in more details.
Remember how the damage received by allies and dealt by enemies works like this: Each enemy has a base preset damage, which is then multiplied by the difficulty factor and by the enemy level multiplier, amongst other things (such as armor penetration %).
Say you have a Venatori Zealot. Say his base preset damage is 50. On routine, since the difficulty multiplier is low (probably 1.0) and the enemy level is low, the outputted damage by the zealot on a basic attack (which has no ability % multiplier) might be close to 50 (before armor). On perilous, since the difficulty multiplier is higher (say 1.2) and the enemy level is higher as well, then the outputted damage might be close to 400. On Nightmare, since the difficulty multiplier is much higher (say 1.5) and the enemy level is much higher, outputted damage might be close to 700.
"Damage reduction" will simply subtract the final output by a % of the base preset damage. What does this mean?
Say you have a 30% "damage reduction" at work. You're playing on routine and get hit by a venatori zealot. Then it's 50 - (0.3 * 50) = 35. Works fine, right?
On Nightmare however, it would be 700 - (0.3 * 50) = 685... Not quite the damage reduction you were expecting.
And yes, you can go over 100% damage reduction and still receive quite a bit of damage.
The way Smothering Veil works is it adds 30% to the initial 15% of Weakened, which adds up to 45% damage reduction. Notice how it ends being ~5-10% damage resistance at most when you actually go ingame in Nightmare and do a received damage data collection