There are level based and other caps on hit/damroll and other things.
A lowbie with insane hr dr would still not hit better then one with just good hr dr.
If you doubt that ac reduced the damage you take get yourself a mob that hits you, then spell your ac up as much as you can (-80 more are easy or just get some armor that does "only" give you ac) then have a look against the same mob again. Besides damage reduction several other combat skills depend on your ac.
At 50 a mix wins, whereon to concentrate most depends on race and class you play.