I noticed that .ar on an armor depends also on .quality

When a player puts on an armor then player.ar differs when an item has diffrent .quality

I really think this is a feature, with what you can more easily create exceptional items that will make the difference. And other stuff like that.

I believe AR is also dependent on .hp/.maxhp so damaged items don't protect/do as much damage (damage on weapons is affected in the same exact way as AR)

Correct, this is a feature. Quality and hp/dmg all affected by Quality so you can have varied quality weapons and armor. Hence the member name