IMO the most elegant solution to this is to give every item in the game, for their level, a random minimum and maximum roll to defenses.
Example to showcase:
level A (30/30) level B (60/60) level C (90/90)
With my suggestion these item could drop as:
level A (15-45/15-45) level B (30-90/30-90) level C (45-135/45-135)
Each stat (phys/magic armor) now has an independent -50% to +50% rolled for each level. What does this mean? If lucky you could find a lvl4 armor that is as strong as a lvl5-6 armor or equal to a lvl7 armor. Or a lvl13 armor that is as weak as a lvl10 armor depending on rolls. Or you could find an armor that has insane phys res for its level but weak magic res for its level.