The flurry perk ONLY affects your dual wield power attack.
And I think the animation for daggers is so fast already it makes no sense to make it faster.
It makes Axes or even Maces hit very fast using dual wield power attack.
Not sure why the last one would not affect daggers though.
Im pretty sure it does, if not its probably a bug.
I am aware that it only affects the dual power attack and I assure you that it is not working (for me), however further testing has resulted in a new finding.
Although dual flurry does not affect double dagger power attacks, if you have a sword in the right and a dagger in the left the dual attack is actually faster than I believe intended as it is near instant.
So in order of speed (fastest to slowest dual power attack); Sword/Dagger, Sword/Sword, Dagger/Dagger. Which seems bssackwards to me.
[Edti fro garmer / adn tpyo]