It's not the same, though.
I disagree. It is effectively the same because it's still a dice roll. An enemy has 45% PA left and you use Knockdown (assuming you won't do enough damage to clear the armor). Can you say, with certainty whether the skill will land or not? The answer is no.
There is no effective difference between a skill with a 45% chance to land a status in a system with no status-blocking armor, and a skill with 100% chance to hit in a system where an enemy with 55% armor left has a 45% chance of being hit with the status. Either way, it comes down to a virtual dice roll.