In another thread, it was mentioned that the masterwork armors can be interpreted not as different materials, but different techniques
What's to say that when you upgrade your +3 leather to +4, you don't have to change up the strapping/rivets/leather conditioning/etc?
The boosts in AC are, as has been stated before, necessary to keep AC in line with your opponent's relative attack power. The way you get to that boost, via differing material, technique, questing, skill challenge, or whatever, is all campaign and DM dependant. Just be sure that you don't penalize characters by making it too hard/lengthy to achieve that AC boost.