Odds are there is a parry/block/avoidance penalty for higher lvl mobs. The same way we need more acc to hit them, we also need more parry (and block rating) to parry/block them.

It may be like with 4 star crafting.
On 3 stars, reducing the difficulty of the recipe by 10 levels did something.
On 4 stars, reducing it by 10 levels does ABSOLUTELY NOTHING.

We may be seeing a similar Player Level VS Mob Level scaling here.