Armor Toughness doesn't change the durability of the armor

I'm trying to make a mod and one of the things that I want to do is armor - the problem is - setting the toughness at 2.0 or higher the durability of the armor is lower than diamonds (2.0). For example, the diamond chestplate is 528 durability (2.0) but the mod chestplate has 400 durability (3.0). Please help!