My previous assumption was that magic armor and magic shields gain an enhancement bonus to their own armor/shield bonuses, which indirectly translates to increased AC. However, in the process of answering this question, I ended up taking a more careful reading of the rules for magic shields, and ran across some inconsistencies in this interpretation. The magic armor and shields section is vague on the subject, only ever stating that magic armor grants an enhancement bonus (without ever specifying where to apply said bonus). So, reading further down the page, we find some circumstantial evidence against my previous interpretation:
Shield enhancement bonuses stack with armor enhancement bonuses.
This clarification is only needed if both enhancement bonuses are being applied to the same value (i.e. directly to armor class). If one is being applied to an armor bonus and the other is being applied to a shield bonus, the stacking rules wouldn't even apply.
A shield could be built that also acted as a magic weapon, but the cost of the enhancement bonus on attack rolls would need to be added into the cost of the shield and its enhancement bonus to AC.
Emphasis mine. There's no mention of an "enhancement bonus to shield bonus to AC," merely "enhancement bonus to AC."
As a result, I'm inclined to believe that magic shields and armor provide enhancement bonuses directly to the user's AC, rather than simply increasing the items' existing armor and shield bonuses. The distinction between the two rules interpretations is relevant in a few edge cases - for instance, in the linked question, a strict reading of the rules for Wild equipment would allow its user to benefit from a shield's enhancement bonus to AC, but not from a shield's enhancement bonus to its own shield bonus.
Is my understanding of these rules correct, and has Paizo ever made any statements clarifying the situation?