From what I understand, the AC of the item plays a role in determining the deflection as well as the location of the strike. Since NPCs don't have skills like Aim and Precise Aim, it is more effective against them. Also Hit Roll helps bypass Armor Deflection. Also the type of attack matters. Something that does a slashing attack striking at armor with a high slash AC is more likely to be turned away.

I thought I read at some point that it is better to match up armor then to have separate types. Ie all Heavy Armor or all Light Armor. I could be wrong there.