"Increased Defense and Protection damage"

Stated in the effects of Rank 9. But it really means the rate is higher, am I correct?

But if that's supposedly true, then why does the summary say its increased at Rank 5?

Maybe, like chain mastery, different races have different potency?

No, it doesn't mean the rate is higher. It means exactly what it says. The debuff can hurt the enemies defense and protection more than before.

At r5 (and r1) the chance for it to happen is higher.

What's the point of saying that if it already says "Can decrease Defense by X and Protection by Y"?

Because it can.