Actually Thought it mirrored any other skill category, i.e. shield: You have an shied cap and shield + contributes to reaching that cap, but Shield Skill + actually increase that cap. This is just my understanding.

There have been rumors of .9 eva + = 1 eva skill + , but if I don't see the stat raise in the menu, then I have to assume that it contributes to reaching the cap and not increasing it.

Given that you now see the effects of geomancy + on dunna, I would now guess that those type of stats are potency increases but I wouldn't be surprised if it a potency increase based on (or scaled to) your actual skill level/amount. For my gil, I go with (+ skill) then I worry about potency, as skill is gonna contribute more to the effect actually happening (or being successful) and potency is gonna effect what happens after it lands. No use for potency if you can't land a certain enfeeb for example. That's just my thought on it.