Are Material Bonuses Implemented Correctly?

My understanding was always that tier 2 mats gave gear both the t1 bonus and the t2 bonus. That is, a Copper Binding (tier 1) gives +WeaponDamage. A Bronze Binding (tier 2) gives +WeaponDamage and +DamageResistance. However, a Simple Leather Strap gives + Dodge, and a Hardened Leather Strap gives +WeaponDamage. Bow strings seem to work like straps (different bonuses for each) but iron sheets work like the bindings (t2 bonus on top of t1 bonus.)

Looking at other things, yards of leather seem to work "correctly." That is, Yard of Simple Leather givea t1 bonus, Yard of Hardended/Supple Leather gives the t1 bonus plus a t2 bonus.

Is this just not implemented correctly/consistently? Or is it not supposed to work like that?