AC bonus is not raised by dex. It does depend on two things: your defense skill, and your agi. At least this has been my experience. Feel free to correct me, if I'm wrong.
Actually it's because 1 point of AC on an item does not add 1 point to the AC stat. I think someone found it to be 1.55 actual AC per 1 AC on an item, though it could be a formula that takes other things into account. 1.55:1 would here though...3*1.55=4.65, which due to rounding could increase AC by 5.
You are all very close. There is no hard and fast formula to express AC value of an item (such as 1.55 multiplied by AC value).
Certain players will get more out of an AC value than others. It is based on your Defense Skill Level. Tanks tend to have high Defense skills because they are often the focus of a mob's attention. That means that an item will more than likely give more AC to a warrior than a caster or even a Rogue.
The Treeweave listed as an example will continue to give increased AC to Tenre Forestcaller as he increases in level (assuming his defense skill increases as well). If you wish to illustrate my example, pass a piece of armor between yourself and someone of lesser Defensive Skill, and record the change in AC.
It is not level based. Trading between an equal leveled Warrior and Mage should illustrate quite nicely, since the Mage will more than likely have nowhere near the same Defense skill.