I think that there's a trend that the oddsmakers see that is similar to last year.
Last year, we slumped mid-season and then began hitting on all cylinders through November. The main reason we didn't lose any games last year was our stellar defense and a healthy Ingram taking the team on his back during the slump.
This season, our defense went through growing pains, Ingram didn't seem to be healthy and we had some injuries on our offensive line. Over the last two games, however, there is definite improvement across the board (evident even against a weak opponent like G.S.). Trent Richardson should be back this game as well.
I think that the oddsmakers believe that this trend indicates that we have improved enough to be basically even with Auburn in this game. Even though Auburn is very good on offense, their defense is pretty average. We should be, outside of LSU, the best defense they've faced. LSU gave up a ton of yards, but only 24 points. We give up, on average, 4ppg (12.8 vs 16.5) less than LSU. Auburn gives up 25 ppg and we average scoring 35.2ppg.
If you take the simplest form of estimation and meet in the middle on the numbers:
Alabama gives up 12.8 ppg while Auburn scores 42.8 ppg: spread is 30 ppg, median is 27.8 points for AU.
Auburn gives up 24.9 ppg while Alabama scores 35.2 ppg: spread is 10.3 ppg, median is 30.05 points for UA.
Add the 3 point home advantage and you get either Bama 33, Auburn 28 or Bama 30, Auburn 25. I'll take either one.