Defense Rating does not = Defense skill. What is listed on the item is the amount of Rating you will get, but the number listed in your paper doll will be the actual Skill. I don't know the conversion numbers though, but it should be easy enough to find: maybe in the Defense and Mitigation FAQ, or WowWiki, something :)
EDIT: I looked it up for you :D
http://www.wowwiki.com/Combat_Rating_System
According to that you need 1.5 Rating at level 60 or 2.37 Defense Rating at level 70 to get 1 Defense Skill.
Edited, Mar 5th 2008 8:14pm by Maulgak