Damage mitigation in LE is simple but complexe to understand.
I like the notion of survivability. It’s easier with it to understand how it works.
Imagine you have A and B that each have 100HP and no protection.
Both take 1 hit for 100 fire damage.
They each went from 100HP (100% HP) to 0HP (0% HP).
Now, A slots an item to get 100HP while B slots an item to get 100 Fire protection.
Both take 1 hit for 100 fire damage.
A goes from 200HP (100% HP) to 100 HP (50% HP) as there is still no mitigation.
B goes from 100HP (100% HP) to 50 HP (50% HP) because of mitigation (100/(100+100)=50% mitigation against fire).
Both loose half there life. Both lose completely there life with 2 hits. Both have the same survivability.
Another way to see mitigation, is there is HP that is real HP and Protection that is virtual HP.
Damage is divided between real and virtual HP proportionaly.
If there is 70% real and 30% virtual HP (for exemple 700HP and 300 Fire protection), 70% of damage is dealt to real HP and 30% of damage is dealt to virtual HP (and so does nothing, is mitigated).
If we increase that a lot, now A has 1000HP and B has 100HP and 900 Fire Protection.
With 1 hit for 100 fire damage, A has 900HP remaining (90%) and B has 90HP remaining (90%).
And both needs 10 hits to die.
If we had C that has 500HP and 500 Fire protection, 1 hit for 100 fire damage leaves him at 450HP (90%) and 10 hits kills him.
They all have the same survivability against fire.
But if they are hit with ice damage, B dies in 1 hit, C dies in 5 hits and A dies in 10 hits.
HP and armour/protection have the same effect but HP is stronger as it is universal. Even more as affixes slots are limited in numbers.
The only way to balance that is by having sources of armour/protection 7 times stronger than HP.
But HP, contrary to armour/protection, has additive AND multiplicative sources.
It’s not easy to compare correctly.
EDIT: I also forgot that we also need to take into account ways to regenerate health that are more effective with lower life, making mitigation stronger.