You're not quite asking the right question here. You don't want to know how armor and rating are related, you want to know why the Orange item has a higher rating at all, if it's got lower stats aside from the net 17 endurance.
The Orange item, as denoted by its color, supports item modifications.
You're right - the base item is a bit of a wash compared to the level 17 green.
But slot in an Armoring mod, a Mod mod, and an Enhancement mod, and then see what comparing the stats again gets ya. It is a higher rank because it has the potential for having better stats - it's just that, due to their nature, Orange items require a bit more investment before they're up to snuff.
The system might also just be favoring the +17 stamina bonus - the system thinks 17 stamina is better than 79 armor.