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Physical damage reduction curve currently isn't correct; they updated it about 3 days ago for Early Access Hotfix #55: "Slightly lowered the amount of Physical Damage Resistance earned per Armor Rating within the upper sweet spot of the curve (~200 Armor Rating.)".

I tested it to double check, and the armor does indeed have some new sort of scaling in the 175-230 region that isn't described above. When I increased armor from 175 to 181, I gained 1.5% reduction instead of the expected 1.8% damage reduction (33% total), and when I increased it by another 15 armor to 196, instead of gaining 4.5% reduction, I only gained 3.7% (36.7% total)