Slice and Dice Bug

So on my rogue, lvl 44, I have the Slice and Dice Rank 2 ability, which is supposed to give 40% attack speed. When I use it, the buff in the top right only says 20% attack speed, and the melee attack speed in my character stats is only increased by 20%. When I tested SnD on a training dummy, it correctly gave me 40% attack speed, but when I went back to fight mobs, it was back to 20%. Am I missing something here?

edit: So it appears that when I use SnD on mobs lvl 31 and lower it gives only 20% attack speed. On mobs lvl 32 and up it gives me the correct 40% attack speed. Still unsure why it does this.