It's more the case of the difference it makes from 90% -> 91% -> 92%. For instance, a base hit of 100,000 is 10,000 damage to 9,000 damage to 8,000 damage at those mitigation values; that is, 10,000 damage to 9,000 damage means you take 10% less damage than before and 9,000 damage to 8,000 damage means you take 11% less damage than before. Mitigation may climb slower at higher Armor and Resistances but the net impact it has grows when removing those last few percentages.
It's the inverse of the +Attribute returns that get smaller per +1% as your total +Damage% grows. +900% -> +1000% just isn't as big of a net gain as +100% -> 200%. Likewise, -90% to -91% is a lot bigger of a deal than -9% to -10%.
Basically, the return of mitigating "1 more percent" is greater as you get more mitigation which balances out the increased cost of how much more Armor/Resist you need to achieve it. That's what creates the essentially linear EHP effect of Armor and Resistance despite their diminishing returns.
Damage reduction from resist and armor is almost completely linear last I checked. (like .01 off). I've known people who could NEVER understand this concept in years of wow. Thankfully they weren't anyone who mattered.
