from the compendium:
Storm Soul: You gain resist 5 thunder and resist 5 lightning. This resistance increases to 10 at 11th level and 15 at 21st level.
While this resistance is active, your arcane powers ignore all targets’ resistance to that damage type up to the value of your resistance.
If you are hit by an attack, you can end this resistance as an immediate interrupt to gain a +4 power bonus to all defenses until the end of your next turn. If you do so, the resistance returns after you take a short rest or an extended rest.
Wild Magic
You draw your spells from the entropic forces of the Elemental Chaos. Whether tapping into the power of primordial beings or drawing strength directly from that plane, you unleash magic in wild surges.
Chaos Burst: Your first attack roll during each of your turns determines a benefit you gain in that round. If you roll an even number, you gain a +1 bonus to AC until the start of your next turn. If you roll an odd number, you make a saving throw.
Maybe better inform yourself before asking questions which are easy to look up. It gives your opinion a bit more weight...
A +1 bonus to AC every other round on average is a whopping +0.5 to AC.
Yeah, that brings the 13 AC up a lot.
The +4 bonus to defenses for an entire round doesn't average +1 AC for the entire encounter either. So, it is still subpar 14- AC on average.
And all of these bonuses are conditional, not automatic. They may or may not be there when the Sorcerer needs them.
There is no way that these tiny or infrequent bonuses to AC are anywhere near the other Strikers AC 17s without taking feats.
Yes, a Cosmic Sorcerer could be in the Phase of the Moon or a Dragon Sorcerer could be bloodied, but those AC boosts are conditional as well. And since not all Cha based races give +2 to Str, it's very possible to be running a starting Cosmic or Dragon Sorcerer with an AC of 13 as well.
The Sorcerer takes Leather Armor and goes from AC 13 to AC 15. Now what? Another feat for Hide Armor?
The Ranger is at AC 16 or 17 before taking any feats.
The Monk and Rogue are at AC 16 before taking any feats.
The Assassin, Avenger and Barbarian are at AC 17 before taking any feats.
The Sorcerer is at AC 13 or 14 before taking any feats. That's an average of 3 lower than the other classes.
You seriously think that these little conditional AC boosts put the Sorcerer in the same league defensively as the other Strikers? You do realize that some other Striker classes have conditional AC or other defensive boosts as well.