Basically at 25k armor you reduce ~60% incoming damage, so a 100k base swing becomes 40k.
at 30k armor you reduce ~64% incoming damage which reduces the previous final swing by 10% to 36k damage. Likewise another 5k armor would give you ~67.6% reduction for another 10% off the previous swing. It's the same reason DRs were given to avoidance. Going from 98% dodge to 99% dodge reduced the incoming damage in half for the cost of 1% dodge. DRs make the gains more even, no matter where you are with the ratings.