In my book that just shows what a mess they made with PHB3, the whole point with masterwork armour was to put the heavy armour wearers ahead on AC, then they go and add unarmored agility and hafted defense to PHB3 to make it more advantagous to not wear armour.
Sorry but PHB3 is just a mess.
Is this observation based on mathematics, or on speculation?
Heavy Armor's purpose is not to be better than light armor, it's to give characters who don't advance their dexterity or intellegence the opportunity to scale their armor as they go up in levels.
Assuming 18 in Dex/Int for light armor....
Cloth is the worst, at 14 AC
Leather and chain are tied, at 16 AC
Hide and scale are tied, at 17 AC
Plate is the best, at 18 AC.
At +4 (which you should start getting by level 14), light armor gives +1 from Masterwork, but you should have increased your primary attribute 4 times, giving another +2. This is a total of +3. Coincidentally, that's the bonus you get from masterwork heavy armor.
At +6 (end game), light armor gives +2 from masterwork, and you have increased your att by 4 for a total of +6. Heavy masterwork armor? +6.
Unarmored Defense only brings you to parity against leather.
Also Hafted Defense? Useful for wizards... yes. But then again, it's also useful for Polearm Gamble builds. And... it does the same ac difference as Shield prof... same bonus type.