There's a bunch of reasons that're all intertwined.
The class (and spells) were from an idea that a friend originally started. But he abandoned it about a year after the start of 3.5 (after some playtesting). I got a hold of it about 2 years later, and I've been working on it off and on since then.
A lot of the spells were originally homebrew or obscure 3rd party, and by the time it got to me, the descriptions were lost. So all I had was the name to go off of when designing spells. In this case, I think granite hide was one that my friend wrote down. Some time later I wrote Wind Armor, and only just noticed the discrepancy when I finally started an Air character for playtest.
I was thinking that at low levels, having +2 AC, or +4 vs nonmagic ranged wasn't a big deal. Then by the time you started seeing +5 compared +10, it wouldn't matter because you'd be seeing very few enemies that hit you with nonmagic arrows. At least, very few enemies that were an actual threat.


Yes, theoretically it can have an infinite AC, but the class and its casting are odd. Spells only go up to 6th level, so there's little desire to reach epic caster levels before you'd want to prestige out. You also get a very low number of spells per day of each level. The class is a bit of a gish in a can.

Granite hide, is an hour per level, but so is mage armor. And GH is a 2nd level spell, with a lower bonus at low levels, and only slightly higher bonus at high levels. *shrug* I don't mind changing the earth one to 10min/level.

You're right, the GH should probably get a bonus effect. Maybe the longer duration is enough to balance out the doubled effect of wind armor. Maybe not. It's why I'm asking for help.

Ok, think I answered all your questions, I feel like I'm starting to ramble. If you want clarification or explanation, don't hesitate to ask.