My guess it that the O-235 cylinders are thinner that the others in the range and have a habit of warping,
Warping would not necessarily cause a wear-ridge BUT if, indeed, these cylinders are thinner, they could well be a more ductile cast-iron "brew" than their more sturdy bretheren.....therefore the rings, in the absence of extra oil-supply could well wear the bore unevenly from top to bottom. It's normal to get a wear-ridge at the top(where the ring stops-top of skirt usually goes a fair bit higher) but the lower-ridge is usually not significantly developed before the upper one dictates a rebore/sleeve/new barrel (dependent on procedure for specific engine.)
Either way, sounds like the design is NOT optimised for longevity and i'd suspect the short-runs/intermittent use nature of GA does it no favours either.
A piston-engine will do loads more if given long stretches at working-temperature. as opposed to "short hops" the cold-start cycle (especially if it's been stood a week and all the internal oil has drained down) causes a disproportionate amount of the wear.
my diagnosis:- poor design, inferior materials.....a good cash-generator for the spares sales business.
cynic?...What! Moi?
