I'm not too sure, I think it's because where they mount isn't perpendicular to the body of the car so they end up looking skewed (even though they're actually level) because you're looking at them on an angle. So from directly above/below it's like
/ ----⠀⠀⠀⠀⠀⠀/ ----
\ ----⠀⠀⠀⠀⠀⠀\ ----
where the \ is the flaps and ---- is the tires. You can see it looking into the wheel well too. It's probably easier to keep them straight when you can mount them squarely to the body and not to the wheel well like the old rally cars did, but I didn't want to drill holes in the body.
Now that I look at it more the spats definitely made it worse too. Ugh.