If you don't believe me, I'll write out the numbers so you can see what I mean ^_^

With Enchanted Armor 's 5% increase on bonus defenses, Zhonya's will provide a base 50 armor and GA will provide 50 armor and 40 MR. For Zhonya's, 50 * 1.05 = 52.5, granting you a bonus 2.5 armor at probably around 10-15 minutes into the game. Then you have GA, 50 * 1.05 = 52.5 armor and 40 * 1.05 = 42 magic resist. So in total, you get a bonus of 5 armor and 2 magic resist. Not so effective :/

Now let's take a look at Block and Unyielding . Block reduces basic attack damage from champions. Again, you are Diana, someone who's early game is weak because of her melee range. Against ranged opponents, they'll easily be able to get 4-5 autoattacks off on your in a single minute. This means that the extra point in Block reduces 5 damage every minute. Laning phase is about 15 minutes, so that's 60-75 damage reduction from poking alone. Now let's talk all engages. Almost all early game fights will result in the enemy laner using autoattacks, because their burst probably won't be enough to kill you outright. Don't you always love that feeling when you away with 5 health? ^_^

Then, you have Unyielding , it essentially functions the same as Block, except it now reduces damage from every source, including abilities. Since abilities have cooldowns, let's be generous and say the enemy pokes you with 2-3 abilities every minute. 15 * 2 = 30, 15 * 3 = 45. So in total, both of these mastery points can reduce over a hundred damage just in the first 15 minutes.