1) Avengers have 18 AC? How do you know this? There's so much assumption in that table it's silly.
Secondly, you might have have high Reflex, but wabbity boobula your Fortitude save is decent! No getting pushed around for you! Reflex tends to be straight damage, but Fortitude tends to be the stuff that puts you in a bad place.
Because there's an avenger character sheet running around the 'net that's really not hard to find. The example character has 16 dex (which could be primary, yes, it's hard to know with the way those pregens are, but as a divine character, my vote would be that the 16 he has in WIS is actually primary), and his feats clearly state: LEATHER ARMOR PROFICIENCY. That's +5, and if you read the sheet, there's a class feature to make up the other +3. I clearly stated all other assumptions: 18 primary, 16 secondary, 1 defensive feat, level 1 characters. Not all of those characters WOULD use the defensive feat, but just to see what would happen if they did, so everyone was on approximately equal footing.
Edit:
Furthermore, I think those assumptions are quite valid. Most characters except for extreme min-maxers go 18/16 in primary/secondary stats. You get weird exceptions with MAD characters like Melee Rangers and Pallys, but they use heavy armor anyway so their stats don't matter much. Some wizards and archer rangers go for 20 in their primary stats, but all that does is push their AC up and the point is to show that the sorc is low relative to other classes, which tend to fall out a little closer, at least relative to role.
The things that stand out to me from that table are the sorc's low AC, the avenger's high AC, and the warden is a little low esp. if he's going to be your only defender. Remember that at low heroic, AC is targetted almost all of the time.