Originally Posted by
Arteezy
Ignore pain provides an absorb (specific value, doesn't change relative to the attack damage).
Other forms of AM are percent reduction modifiers from damage (effectiveness scales with damage, more damage 'absorbed' from the DR).
With raid bosses doing more damage, the difference between the two changes drastically. As the damage per swing increases infinitely, a flat DR gains more increasing value. For example, 100k absorb vs 1% DR, in a time frame of 1 single melee attack. 100k swing vs 10mil swing.
1E5 swing:
100k absorb = 0 dmg taken
1% DR = 99 dmg taken
1E7 swing:
100k absorb = 900k dmg taken
1% DR = 900k dmg taken
This is obviously an oversimplified and exaggerated case to help you understand the logic. But hopefully it helps create an understanding of how the different tanking models will behave in different environments.