In the SRD it states:
Protection This ring offers continual magical protection in the form of a deflection bonus of +1 to +5 to AC. Faint abjuration; CL 5th; Forge Ring, shield of faith, caster must be of a level at least three times greater than the bonus of the ring; Price 2,000 gp (ring +1); 8,000 gp (ring +2); 18,000 gp (ring +3); 32,000 gp (ring +4); 50,000 gp (ring +5).
Compare that to:
Caster Level for Weapons The caster level of a weapon with a special ability is given in the item description. For an item with only an enhancement bonus and no other abilities, the caster level is three times the enhancement bonus. If an item has both an enhancement bonus and a special ability, the higher of the two caster level requirements must be met. http://www.d20srd.org/srd/magicItems/magicWeapons.htm
So according to a strict reading when detect magic is cast a ring of protection always radiates faint abjuration (CL 5th) and the creator as a perquisite for the crafting simply needs to be of a caster level at least three times greater than the bonus of the ring created. Regardless of the actual deflection bonus of the ring, a detect magic will always radiate faint abjuration.
Whereas conversely a weapon with a magical enhancement bonus to hit and damage always has a caster level (and perquisite) three times the enhancement bonus and will 'register' as such when a detect magic determines the strength of the aura.
This would seem to be a slight but significant difference in the way these magical items are handled in respect to detect magic. Is that anybody else's understanding of this nuanced point or does anybody have a different interpretation?