Mana Reflection reflecting 5% rather than 20% of max hp

Describe the problem. Be as specific as possible!:
Mana Reflect says its supposed to do 1000% of the dmg taken back to the monster but it can't be more than 20% of the monsters max hp.

I went to Oblivion Guardians to test it and noticed it was only doing at most 5% of the monster's max hp
(They have 370,000 hp, I take 2513 dmg but only reflect 18,500 at most. I should have reflected 25,130?)

I also tested this with all my other defensive buffs off and the same thing happened.

Is there something else affecting this that I'm not aware of?

 Images of problem