2 sides of the same die

Usually the leather is tempered/boiled for extra strength

That makes it less flexible though, and even leather that’s been treated like that still offers extremely little protection, not really more than just thick sturdy clothing
Leather armour is not armour

There's a reason base leather armour just adds 1 to a D&D character's base AC, but allows for the full Dexterity modifier to be added and doesn't give disadvantage on stealth rolls, unlike chainmail or heavier armours.

I’m not talking about fantasy settings, I’m talking about our own world. Leather armour was used rarely, if ever, simply because the material is too costly for the appalling protection it provides