further examination of sundering a ring
Attacking an object is different than attacking a creature. Armor rules are different than cover (except in the case of the tower shields). These systems are completely separate.
Why give the ring cover?
Cover is defined as any barrier between an attacker and defender. Such a barrier can be an object, a creature, or a magical force. Cover grants the defender a bonus to AC.
For this example, sundering a ring, the defender is the ring. The ringbearer's fingers (the ones next to the finger wearing the ring), give the ring cover - they are a physical barrier between the attacker and defender. Simply making a fist gives the ring over 50% cover.
It would be just like having a friend on either side of you. Now if someone attacks from directly in front of you, you will have no cover (note that the ring still gets cover because the other fingers are tiny and occupy the same space). However, if the attacker was adjacent to you, one of your friends (the one on that side), could provide cover to you.
Where it really gets weird is if the character knows you are going for the ring so he waves his hand about wildly back and forth. Would this give a blur effect and subsequently a 20% miss chance due to concealment?