A misunderstanding of Neverwinter stats system

All DR that is not shown in your stats are separate layers. Your DR from defense is called basic stats, just like the basic damage stats of your weapon. If you see some item that will add 10% or 20% DR to your currently DR, that means there will be a proportional buff. For example if you have 50% DR as your stats, and after the additional buff from negation (40%). Then it will be 50*(1+0.4)=70 DR, so enemies need about 175 RI to fully penetrate you (175*0.4=70).
Same thing applies to the nerf system. If you see something that will nerf your armor pen by 40%, that means it will make their AP like AP*0.4.
It’s a little bit confusing but yeah that’s how Neverwinter works.