Its very cotradictive in the last two sentences of the description " This trait does not stack with armor. This armor stacks with worn armor, but not the machine trait." I get the whole machine trait deal... But the worn armor is what's getting me.

At the risk of having someone jump me I would say natural armour stacks with any other armour 'worn' on that location. The machine trait I agree shouldn't stack with natural armour since they are broadly speaking of the same category (armour that is part of your character i.e. doesn't typically get removed).

That of course is simply one way of playing it. Some will say no armour stacks and simply award the highest armour bonus to the location. Both approaches demonstrate some merit. As for the contradiction, I think it is safe to assume the two cancel each other out, and it does in fact stack with armour 'worn' on that location; unless there is a future errata that states otherwise I wouldn't worry about it.

As a GM your ass would be covered either way (conveniently enough) as a player its really up to the GM. Good luck, hope it helps but I have a feeling its just going to be a preference. No one really wants armour stacking to be an issue, but realistically if there was going to be an exception to the rule this is the situation.