User blog comment:JBox2D/Sora the Sunrider/@comment-7709681-20140706012624/@comment-24090662-20140706043312

It's not well worded, my bad.

Basically, the total damage reduction with max stacks is 100% at each level, but the number of stacks improves per level. The reduction per stack goes down because the stack limit goes up. Let's use the example of someone taking 5 instances of 100 damage with each level of the passive.

Level 1: One stack, 100% reduction per stack. Take 100 damage, 0 stacks. Total damage taken is 400.

Level 2: Two stacks, 50% reduction per stack. Take 100 damage, reduced by (50% * 2), lose one stack. Take another 100 damage, reduced by (50% * 1), lose one stack to 0 stacks. Total damage taken is 350.

Level 3: Three stacks at 33.3 reduction each. 100 damage reduced by 100%, lose a stack. 100 damage reduced by 66.6% = 33.3 damage, lose a stack. 100 damage reduced by 33.3% = 66.6 damage, out of stacks. total damage is 0+33+66+100+100 = 299 damage.

Level 4: 4 stacks, 25% reduction per stack. Take 100 damage, reduced by 100%, lose one stack. Take another 100 damage, reduced by 75% = 25 damage. Take another 100 damage reduced by 50% = 50 damage, lose one stack. Take another 100 damage reduced by 25% = 75 damage, lose one stack. Total damage taken is 0+25+50+75+100 = 250 total damage taken.

Level 5: 5 stacks, 20% reduction per stack. Take 100 damage reduced by 100%, lose a stack. Take 100 damage reduced by 80% = 20 damage, lose a stack. Take another 100 reduced by 60, then 40, then 20. Total damage taken is 0+20+40+60+80 = 200 damage.

It looks like it goes down per level because the stacks go up. The actual damage mitigated goes up per level. The reduction per stack would only matter if she had an ability which granted her a certain number of stacks independent of the maximum, where levelling would reduce it's usefulness.