You describe inner walls and not the shoulders or inner edges of tread wearing, which says to me the tyre inside is rubbing against something as the suspension moves, could be suspension leg or inner wheel arch or even inner lining soundproofing sagging when wet, though there ought to be some polished or worn surface to show where things are catching.

If i have read the post wrongly and it is indeed the inner edges, then are you keeping a close eye on pressures.

If there is no obvious suspension fault by examination, it's sometimes revealing to follow your own car over a varied journey and watch closely how the suspension behaves, ie what might appear to be sound shock absorbers are in fact allowing the tyres to bounce up and down constantly on the road...something i observed on Saturday whilst keeping a very decent distance from a poorly driven Corsa on the M6 roadworks near Coventry...also if the car is crabbing sideways that is not easy to spot unless you see it being driven.

If it's just a characteristic of the car and no obvious faults, then it would make sense to fit symmetrical (non directional) tyres when you replace and to rotate them regularly (maybe getting them turned on the wheel at half life point) in order to get some wear out of the rest of the tread width.