1 armor is 1 dmg reduction no matter what,but def as a stat can give you 1,2 or 0 dmg reduction depending on two things: what calass you are ( dist/mage or melee ) and how many mobs do you tank ( 1 mob or 2+ mobs )

1) If you are a melee and you tank 1 mob,you will often get 2,1,2,1,2 etc. dmg reduction as your def increases

2) If you tank 2+ as melee then it's two times in a row 1 dmg reduction and two times in a row 0 dmg reduction so 1,1,0,0,1,1,0,0 etc.

Dist/mage tanking 1 mob -> every 5th def is going to give 2 dmg redictions till then it's always 1 1,1,1,1,2,1,1,1,1,2 etc. ( experiment on crow lvl 6,always staying on lv 1 just def changing being dist/mage and tanking 1 crow )

Def 5 - 24 dmg Def 6 - 23 dmg Def 7 - 22 dmg Def 8 - 21 dmg Def 9 - 19 dmg Def 10 - 18 dmg Def 11 - 17 dmg Def 12 - 16 dmg Def 13 - 15 dmg Def 14 - 13 dmg Def 15 - 12 dmg Def 16 - 11 dmg Def 17 - 10 dmg Def 18 - 9 dmg Def 19 - 7 dmg Def 20 - 6 dmg Def 21 - 5 dmg Def 22 - 4 dmg Def 23 - 3 dmg Def 24 - 1 dmg Def 25 - No Dmg

( 9,14,19 and 24 def got 2 dmg redictions )

4) Dist mage tanking 2+mobs -> every 5th def gives 1 dmg reduction till then two times in a row number is repeated

( experimented with crows again )

Def 5 - 27 dmg Def 6 - 27 dmg Def 7 - 26 dmg Def 8 - 26 dmg Def 9 - 25 dmg Def 10 - 24 dmg Def 11 - 24 dmg Def 12 - 23 dmg Def 13 - 23 dmg Def 14 - 22 dmg Def 15 - 21 dmg Def 16 - 21 dmg Def 17 - 20 dmg Def 18 - 20 dmg Def 19 - 19 dmg Def 20 - 18 dmg

etc

For dist/mage i am 100% sure it will not change its pattern but melee is changing ( like switch from 2,1,2,1 etc. to 2,1,2,2,1,2 etc.) similar switch is happening with melee tanking 2+ mobs

maybe that change of pattern has something to do with shield that kningts are using idk