Leech not working according to my math

Hi

I'm playing a monk with mana leech to shield, and as I read the abilities I'm not getting what I'm supposed to.
I'm running with vulnerabiliy to the mobs, level 62 mobs, and I have 19,2% mana leech from physical attacks with tempest furry, and the ability states that it does between 211-435 physical damage, I'm seeing that I'm getting 30 or less shield each attack, but according to my math it should be closer to 40 shield minimum, I know there is still some armor that it probably has to account for, but it feels like I'm getting low amounts, it works over all, but it feels like the math is off.

Keep up the good work <3