Why upped items have varies result.

I am tired of people making up their own theory. Let’s take Herald Of Zakarum
Gilded Shield for example.

Gilded Shield is an Exceptional Unique Armor, the defense can range from 144-168. But since it's a unique item (Herald Of Zakarum), it will always be perfect 168 when it's found (unidentified). So if you found a 180% Hoz, you will have a 168x1.8+168 Defense Hoz which is 470.

Now when you up the Gilded Shield Hoz to a Zakarum Shield Hoz, the result can be varies. A Zakarum Shield has defense range of 169-193. So that's 1-25 more defense than your Gilded Shield. When you up your Gilded Shield, the result varies depend on your luck, if you are unlucky person and you upped a 169 Zakarum Shield Hoz, 169x1.8+169=473, your shield will only have extra 3 defense.

This applies to most unique items. So when you up something and you only get +2 defenses, it's because you are unlucky, so suck it up like a man.

B.T.W: if you really care about that couple defense, you should use rollback method.