I've noticed over the course of a few characters that aim/precise aim go up the most when fighting mobs with shields. I thought these skills were meant to work against any kind of worn armor. Why is this?

The difference in the amount of skill improvements received (shield v armor) is so noticeable to me that it makes me wonder if the skills are actually firing or do much against worn armors.

To give an example, I think it was Magner I was playing and I was shooting to get an edge but I needed those skills as a prerequisite. I at first tried fighting mobs with armor and saw little improvement. But when I started fighting mobs with shields the skills improved quite a bit.

Really simple.

The mob's shield block skill calls against your aim etc.

On a related note, that's why you can also improve armor use skills much faster through using a shield of the relevant type than just through actual armor deflection.

My guess is that the code requires you to have a "success", but success for a shield block involves just boosting the block (or maybe a successful shield block after the successful boost, which isn't that rate), whereas for the armor it has to actually deflect after?

You believe that mobs get the shield block skill. And in order for them to block successfully it has to check against your aim skill, which can cause the improvements.

On the flip side, mobs (or most) do not have the armor deflection skills so they can't block your blow with their breastplate, which does not check against your aim, therefore no aim improvements?

Does that sum it up?

Cavaet: I am not a coder.

Mostly it and amounts to you need an armor deflection attempt to improve aim/precise aim. Shields are included as armor, but while most armor has a low chance of deflection, if you're dealing with shield block skill, every successful shield block uses armor deflection
, giving a chance to try aim.

As to mobs specifically, I believe both still apply, though personal experience is similar that mobs stink at armor use.

This.

Pretty sure what you are saying is what I was meaning.

Mobs have both skills. Just that one will normally fail a check.