There is a difference between wear and growth and we have to keep that in mind. While I agree that if all is straight the hoof will wear evenly, that doesn't mean that it will grow evenly and I think that has a bearing on what we perceive as growth and also the resulting splay in or out.
The only growth differences should be the amount of heel vs the amount of toe that a horse grows based on 'genetics'. Lateral growth will be equal if all is right in the horse.
I am 99% certain, for example, that my friend's paint colt grows his front hooves unevenly. The first time I trimmed him, his heels were off, so I evened them up and my friend was very impressed with how much better he stood and moved after that. So I went again in 6 weeks and again his heels were uneven. He was on soft pasture and I doubt the uneven heels were a result uneven wear.
I will bet money the horse is not symmetrical and moves crooked and that is why you see the change in the heels six weeks later. If the horse moved straight, was evenly muscled, that wouldn't happen.
Unbalanced feet can cause body unsoundness. Body unsoundness can cause unbalanced feet. Fix the feet, fix the body...no more issues regardless of the horse toeing in or out, or whatever...again, except in the cases where the horse is not mechanically functional.
Another way of saying it, 'make the world (hoof capsule) right with the frog'. The frog is static and is at the end of that crooked or straight limb, therefore it is the frog that dictates how much heel, how much toe, how much wall, and therefore balance.
If the farrier looks at that foot, and that foot isn't directly below a relaxed free falling limb when he looks, the farrier will never get that foot balanced, no way, no how, and that is the biggest mistake made.