funny place this sometimes si.
you break the bore in with the first ring, the bore will have very small machine lines that get knocked off with the first ring.
this can ware the ring slightly. so you replace the ring! you have to knock the tops off these machine marks but not polish the bore, therefore you dont let it tick over, and over heat, polishing the bore or even worse seizing the engine.
thats how i had it explained to me anyway, no doubt others will have different ideas.