Doesn't matter because the BCS is the agreement that all of the teams signed on to. The AP is just a third party group of guys that votes.
The AP had USC ahead of Texas much the year, yet they were wrong. The BCS is the college version of a playoff system - it's not perfect but it is the system that Southern Cal and Auburn agreed to.
Last year, Auburn would have had a good claim to the national title under the old system but they signed on to the BCS to they have no claim no matter what some polls might say. The same goes for USC in 2003.
Correction. It's the agreement that SIX conferences agreed to, and it's far from perfect. The reason for this agreement? Well, of course, greed and money. The six conferences would have to share the money (although a bigger pot) with schools outside of those conferences, instead of getting the guaranteed money they currently receive and is kept away from the other schools. No excuses should be made for the lousy system we currently have with BS (pun intended).