Why Tanks like Mundo, Rammus and Malphite became stronger with 5.22.

Last Whisper and the items it builds into only effects bonus armor

BotRK no longer counters tanks as well as it did. that 2% current health might seem meh but it does add up.

The % damage reduction keystone

How does the armor pen change impact the game? I'll give an example of armor effectiveness in 5.21 VS 5.22

5.21: level 18 mundo has 86 base armor and 100 bonus armor Ashe has Last Whisper and the % armor pen mastery

armor pen would be in this case leave (1*(1-.35)(1-.06))= 0.611 % of the armor on the target

This in turn means that mundo has (100/(100+(86+100)*0.611))= 100/213.646 or 46.80639937% damage taken or 213.646% more effective health

5.22: level 18 mundo has 86 base armor and 100 bonus armor Ashe has Lord Dominik's and the 7% armor reduction from masteries

100/(100+(86+(100*.6))*.93) = 100/235.78= 42.412418356% of the damage is taken or an effective 235.78% more health

While the new one starts preforming almost as good at 200 bonus armor (5.21: 36.4% of the damage is taken VS 5.22: 34.3% of the damage is taken.) it takes a total of 518 bonus armor for the item to catch up with the old LW in the given situation.

This is the math behind why tanks became stronger with 5.22.

If it ain't broke don't bother to try and fix it.