Mage Arcane Shield

[Turbobasher]

I was curious about the damage reduction for arcane shield.

That's 55% for charged


That's an extra 15%


Another extra 15%


And a final 15%

If u add them up that's 55+3(15)= 100% damage reduction. But I know for a fact that it doesn't absorb 100% damage. So do these numbers stack multiplicatively or additively. If they do in fact stack additively why doesn't it reduce damage by 100%?
Thanks