if you check out the specs on Rings of greater protection in map editor they give this bonus.
Ring of greater armor
<arch path="magic/ring/prot_ring">
<string name="id" value="Ring of Greater Armor"/>
<string name="id-desc" value="This ring protects the wearer against melee damage."/>
<int name="bonus-ac" value="3"/>
<int name="value" value="15000"/>
<int name="gen-freq" value="64"/>
<int name="gen-level" value="8"/>
<int name="wear-level" value="7"/>
</arch>
Ring of Greater Protection
<arch path="magic/ring/prot_ring">
<string name="id" value="Ring of Greater Protection"/>
<string name="id-desc" value="This ring protects the wearer against all types of damage."/>
<int name="bonus-protection" value="2"/>
<int name="value" value="32000"/>
<int name="gen-freq" value="64"/>
<int name="gen-level" value="12"/>
<int name="wear-level" value="9"/>
</arch>
so therefor a ring of greater protection gives bonus to a "protection" stat which is a side thing to armor. (I'm just taking a stab at this, so I may be wrong.)