I never researched to find the why with the varsol. It was the only possibility in his case though.

I did at the time hear an automotive machine shop verify it was the cause of the problem. It is a very common solvent used in many parts washers I think. .

What rings stick first may be more dependant on the actual material used in manufactuing them. Some are cast iron or used to be and some are perhaps cast steel. Perhaps when a set is purchased their composition is different.

Also what end of the piston sees the most atmospheric moisture. May be a factor. I consider it almost total death to store an engine long term in a damp location unless really protected. If those aluminium pistons become oxidized it would be like putting a grinding compound in the engine. Aluminium oxide is very hard.

I too hope some chemist type might enlighten us about what varsol is actually composed of that could account for this problem. I really do not have a clue why it does this myself.