I tried to a run a test of which one outperforms the other per point, and I ran into a problem. I picked 2 pieces, One has +3 deflection, the other has +3 defense. Both have no AGI.
Dragonscale Gorget VS Tynnonium Shackle
Switching them out did absolutely nothing. 3 defense switched for 3 deflection did not change the numbers in any way. Removing either one completely was actually taking down the other stats not even present on the wrist piece to begin with. I even stayed out of stance to make sure i wasn't even close to the soft caps. I'm sorry I can't calculate the actual number, but it appears theres another broken mechanic at work
The only test i could perform was switching from no stance into stance. Going to full tank stance which for me is +28 def/deflection defense netted a whole .2% over deflection.
If I'm not mistaken the uncontest part is added on after the contested check, but will NOT show up in the persona window, just like picking up the + ripostle % in the strength line wont show up under parry(or have they fixed that? been a long time since i played with the STR line).
Honestly I think that if it simply made 16% of what you already had uncontested it would absolutely retard the point vs a solo mob, because in that battle you would have your actual avoidance as stated, and there would be no point to ever using defensive in that battle. In the case of mobs under us in levels it would hurt us because it would make 16% static deflection, whereas with the mob being say green to us it would multiply time whatever the contested formula is and probably produce about 25-30%. Its gotta be an additional.