I can see your logic. I still believe that cover is a visual modifier, and so should apply to anything that concealment and darkness would apply to. You're right-- It doesn't make a lot of sense that being partially behind a wall would make it easier to resist believing an illusion. But neither does it make a lot of sense that if an area is dark, it's harder for you to cast Stealth.
But it does, because all spells require Line of Sight, and as the book says, "Spellcasting by visual targeting is subject to normal visibility modifiers." (page 281, Step 2: Choose The Target). Someone can argue that seeing any part of someone's aura is as good as seeing all of it, but that's conjecture-- How casting a spell works in-universe is not so defined, as far as I know. After all, if it were that binary, what would the Spellcasting roll or the Resistance roll represent? The same way that you can fail to cast a spell, and that someone can resist it completely, I think how much aura you are able to aim your magic at affects the overall outcome of the spell. Maybe when you have LOS to less of their aura, you aren't able to get as much of your mojo into them, which is represented by penalties to your roll or bonuses to theirs. The dice are abstractions after all.