I remember the same being true of all the auras, at least the ones with AC. The spell description is that it adds much more AC than it actually adds (can't log in to verify exact numbers). It was mentioned somewhere on the forums (though I can't find it, search feature won't add +ac to my search

) a while back, likely several times, and some answer was given though I forget what it was.
This error is also in the description the aura NPC gives you of each aura. The stats reported from what the npc says, what wiki says, what mq2 says (from reading description), and what #mystats says vary to differing scales on a number of stats on a number of auras. I'd log in and write down the exact discrepancies, but I'm very sleepy

Plus I believe the NPC only tells you the stats of the auras upon completion of the quest when you select which one you want, which is a quest I'm not going to repeat

In the end, it doesn't matter a whole heck of a lot. Due to the dynamic of the auras, you're going to select the aura you select based on what stats they add, not the scale of the stats added. And as I'm pretty sure it's been noted before, you're very likely getting the correct stats from the aura, it just looks funny in the spell description.