Why does removing the SSA on the last fire field in POI negate the dmg?

Question

Is it a mechanic or something with the damage calculation?

field 01: 300 fire damage + 12.5 physical damage = 312.5 total damage
field 02: 300 fire damage + 25 physical damage = 325 total damage
field 03: 300 fire damage + 50 physical damage = 350 total damage
field 04: 300 fire damage + 100 physical damage = 400 total damage
field 05: 300 fire damage + 200 physical damage = 500 total damage
field 06: 300 fire damage + 400 physical damage = 700 total damage
field 07: 300 fire damage + 800 physical damage = 1,100 total damage
field 08: 300 fire damage + 1,600 physical damage = 1,900 total damage
field 09: 300 fire damage + 3,200 physical damage = 3,500 total damage
field 10: 300 fire damage + 6,400 physical damage = 6,700 total damage
field 11: 300 fire damage + 12,800 physical damage = 13,100 total damage


so why does REMOVING the physical protection (SSA) result in only 300 dmg taken?